This abstract class contains most of the fields (aside from the id and
foreign key to StudentModule that the subclasses need to manage).  It
also provides a get_history method that abstracts searching across
multiple backends.

Move router code to openedx/core
We need to use it from cms and lms.
Ensure aws_migrate can be used for migrating both the lms and cms.

Handle queries directed to student_module_history vs default and the
extra queries generated by Django 1.8 (SAVEPOINTS, etc).

Additionally, flag testing classes as multi_db so that Django will
flush the non-default database between unit tests.

Further decouple the foreignkey relation between csm and csmhe

When calling StudentModule().delete() Django will try to delete CSMHE
objects, but naively does so in the database, not by consulting the
database router.

Instead, we disable django cascading deletes and listen for post_delete
signals and clean up CSMHE by hand.

Add feature flags for CSMHE
One to turn it on/off so we can control the deploy.
The other will control whether or not we read from two database tables
or one when searching.

Update tests to explicitly use this get_history method rather than
looking directly into StudentModuleHistory or
StudentModuleHistoryExtended.

Inform lettuce to avoid the coursewarehistoryextended app

Otherwise it fails when it can't find features/ in that app.

Add Pg support, this is not tested automatically.

This is a clone (copy) of CSMH's declaration and methods with an added
id of UnsignedBigInAutoField

We should be able to delete the save_history code, but needs testing.

Add error logging when capa failures happen

Put StudentModuleHistory into its own database

Bump out the primary key on CSMHE

This gives us a gap to backfill as needed.
Since the new table's pk is an unsigned bigint, even for people who don't
consolidate CSMH into CSMHE, the lost rows are unlikely to matter.

Remove StudentModuleHistory cleaner

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

Remove duplicated course listings template code on the student dashboard.

Original Commit Messages:

use edx's own get_parent method, rather than our own.

add field to unique constraint to avoid MultipleObjectsReturned in case of multiple browser clicks on submit

fix 0011 migration, inherit from TimeStampedField and add composite index (migration only)

fix bug where adding an already registered user to a ccx would cause a crash due to an undefined variable

add assertNumQueries tests to test modules where override field providers are used

remove unnecessary teardown

implement recommended style for checking empty list

import utility methods rather than use duplicate code

added comment explaining date conversion to string for json

add logging for invalid users or emails when enrolling students

add comment about xmodule user state

avoid using get_or_create, which seems to be causing a race condition on schedule change save

relocate badly placed edvent handlers to fix multiple submit problem

individual students, and a reimplementation of the individual due date
feature.

This work introduces an architecture, used with the 'authored_data'
portion of LmsFieldData, which allows arbitrary field overrides to be
made for fields that are part of the course content or settings (Mongo
data).  The basic architecture is extensible by means of writing and
configuring arbitrary field override providers.

One concrete implementation of a field override provider is provided
which allows for overrides to be for individual students.  This provider
is then used as a basis for reimplementing the individual due date
extensions feature as a proof of concept for the design.

One can imagine writing override providers that provide overrides based
on a student's membership in a cohort or other similar idea.  This work
is being done, in fact, to pave the way for the Personal Online Courses
feature being developed by MIT, which will use an override provider very
much long those lines.

Rename fields in CourseTaskLog, including a task_key column for indexing.  Use 'rescore' instead of 'regrade'.  Clean up task submission.

from client.  Tasks.py remains the task implementations running on the
celery worker.

In particular, move status message generation out of task thread to client side.

Add CourseTaskLog, and make calls to regrading.

Add regrading implementation, and hack the marshalling of request.

Switch timed_module to store location, and use to navigate from timer when timer displays on non-exam course pages.

* reorder index fields

* Update all uses.