Let staticfiles determine the footer URLs

For a smoother transition to the new world

Having ENABLE_S3_GRADE_DOWNLOADS enabled by default in common.py
could lead to surprising behavior for folks downstream. They'd
suddenly see a grade download screen on their new instructor
dashboard, but the links by default would be local files and
couldn't be used in an actual production environment. So we disable
by default and let people explicitly enable it and set it up for
S3 if they wish.

LMS-58

that are not present in a given student's gradeset.

General code cleanup and addition of comments.

Instructor dashboard API unit tests.

LMS-58

Restrict grade report generation to 'is_superuser' users (can be overridden with
feature flag ALLOW_COURSE_STAFF_GRADE_DOWNLOADS); all staff users can download
generated files.

LMS-58

Hook up display of grade files ready for download to new instructor dashboard.

LMS-58

The input string is not expected to contain any unicode characters

squashing to one commit to make cherry-picking by feature possible

LMS-1355

Fixed pep8/pylint violations

Added email feature flags; use full names for pylint disables

add DOC_STORE_CONFIG to test settings

Adds xblock handler_url support to the LMS, and makes handle_ajax use
that code.

[LMS-230] [LMS-229]

Allow to control which of the optional fields from the registration page
are being displayed to the user. Added the configuration variable
`REGISTRATION_OPTIONAL_FIELDS` to control which ones. The default value
shows all the fields as currently; to not display some of the fields,
set the configuration variable with only the fields you want to display:

```
REGISTRATION_OPTIONAL_FIELDS = [
    'level_of_education',
    'gender',
    'year_of_birth',
    'mailing_address',
    'goals'
]
```

LMS-1244

LMS-1418

Add Pending Tasks section to the following tabs:
* Course Info
* Student Email
* Data Download
* Email

LMS-1242

Bug: LMS-1363

Usefull variables for translation