Add app_label definitions in Meta classes for models.
Move import of course_mode model into methods to prevent importing
the models at startup time.

Cached values were leaking across tests, causing difficult to debug errors,
particularly when using Config Models. As part of this work, certain tests
that had query counts that relied on those values being cached needed to
be adjusted up.

ECOM-3354

Two new certificate statuses are introduced, 'audit_passing' and
'audit_notpassing'. These signal that the GeneratedCertificate is not
to be displayed as a cert to the user, and that they either passed or
did not. This allows us to retain existing grading logic, as well as
maintaining correctness in analytics and reporting.

Ineligible certificates are hidden by using the
`eligible_certificates` manager on GeneratedCertificate. Some places
in the coe (largely reporting, analytics, and management commands) use
the default `objects` manager, since they need access to all
certificates.

ECOM-3040
ECOM-3515

This reverts commit 60860e3a.

An `eligible_for_certificate` field is added to the
GeneratedCertificate model. This way we can retain existing grading
logic, as well as maintaining correctness in analytics and reporting.

Ineligible certificates are hidden by using the
`eligible_certificates` manager on GeneratedCertificate. Some places
in the coe (largely reporting, analytics, and management commands) use
the default `objects` manager, since they need access to all
certificates.

This commit also updates the DB cache for acceptance tests.

An issue arose recently due to ATOMIC_REQUESTS being turned on by default. It
turns out that CohortMemberships had been somewhat relying on the old default
transaction handling in order to keep CohortMemberships and the underlying
CourseUserGroup.users values in-sync.

To fix this, I've made all updates to Cohortmemberships go through an
outer_atomic(read_committed=True) block. This, is conjunction with the already
present select_for_update(), will no longer allow 2 simultaneous requests to
modify objects in memory without sharing them. Only one process will be
touching a given CohortMembership at any given time, and all changes will be
immediately comitted to the database, where the other process will see them.

I've also included some changes to get_cohort(), add_user_to_cohort(), and
remove_user_from_cohort() in order to properly make use of the new
CohortMembership system.

ECOM-2972

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

Updating previously-existing unit tests to function with the new
CohortMembership model.

Also remove useless docstrings where they were added to keep pylint
quiet.

TNL-3204

TNL-3179

problem from legacy instructor dash to new instructor dash.

The progress page did a number of things that make performance terrible for
courses with large numbers of problems, particularly if those problems are
customresponse CapaModule problems that need to be executed via codejail.

The grading code takes pains to not instantiate student state and execute the
problem code. If a student has answered the question, the max score is stored
in StudentModule. However, if the student hasn't attempted the question yet, we
have to run the problem code just to call .max_score() on it. This is necessary
in grade() if the student has answered other problems in the assignment (so we
can know what to divide by). This is always necessary to know in
progress_summary() because we list out every problem there. Code execution can
be especially slow if the problems need to invoke codejail.

To address this, we create a MaxScoresCache that will cache the max raw score
possible for every problem. We select the cache keys so that it will
automatically become invalidated when a new version of the course is published.

The fundamental assumption here is that a problem cannot have two different
max score values for two unscored students. A problem *can* score two students
differently such that they have different max scores. So Carlos can have 2/3 on
a problem, while Lyla gets 3/4. But if neither Carlos nor Lyla has ever
interacted with the problem (i.e. they're just seeing it on their progress
page), they must both see 0/4 -- it cannot be the case that Carlos sees 0/3 and
Lyla sees 0/4.

We used to load all student state into two separate FieldDataCache instances,
after which we do a bunch of individual queries for scored items. Part of this
split-up was done because of locking problems, but I think we might have gotten
overzealous with our manual transaction hammer.

In this commit, we consolidate all state access in grade() and progress()
to use one shared FieldDataCache. We also use a filter so that we only pull
back StudentModule state for things that might possibly affect the grade --
items that either have scores or have children.

Because some older XModules do work in their __init__() methods (like Video),
instantiating them takes time, particularly on large courses. This commit also
changes the code that fetches the grading_context to filter out children that
can't possibly affect the grade.

Finally, we introduce a ScoresClient that also tries to fetch score
information all at once, instead of in separate queries. Technically, we are
fetching this information redundantly, but that's because the state and score
interfaces are being teased apart as we move forward. Still, this only
amounts to one extra SQL query, and has very little impact on performance
overall.

Much thanks to @adampalay -- his hackathon work in #7168 formed the basis of
this.

https://openedx.atlassian.net/browse/CSM-17