Modify CourseOverviews to create course image thumbnails.

Course teams occasionally upload very large files as their course
image. Before this commit, those images would be used directly in
the student's dashboard, sometimes leading to MBs worth of image
data on that page. With this commit, we now auto-generate small
and large thumbnails of configurable size. The Student Dashboard
and Course About pages will make use of this new functionality
(CourseOverview.image_urls), but the behavior of
CourseOverview.course_image_url will not change.

Note that the thumbnails are still created in the contentstore,
and sit alongside their originals.

What's included:

1. Multiple sizes, currently starting with "raw", "small", and
   "large". This falls back to the current behavior automatically in
   the case where thumbnails don't exist or this feature has been
   disabled in configuration.

2. Django admin based configuration for image sizes and whether
   to enable the functionality at all. Note that to regenerate
   images, you'd need to wipe the CourseOverviewImageSet model
   rows -- it doesn't do that automatically. This is partly because
   it's a very rare operation, and partly because I'm not entirely
   sure what the longer term invalidation strategy should be in a
   world where we might potentially have multiple themes. The
   flexible configuration was intended to allow better customization
   and theming.

3. The Course About pages also use the new thumbnail functionality,
   as an example of "large". This is in addition to the "small"
   used on the student dashboard.

Things I'm punting on for now (followup PRs welcome!):

1. Bringing the thumbnails to course discovery. A quick attempt
   to do so showed that it wasn't getting properly invalidated
   and updated when publishes happen (so the old image still showed
   up). It probably has something to do with when we do the
   re-indexing because it stores this data in elasticsearch, but
   I'm not going to chase it down right now.

2. Center-cropping. While this is a nice-to-have feature, the
   behavior in this PR is no worse than what already exists in
   master in terms of image distortion (letting the browser handle
   it).

3. Automated invalidation of the images when a new config is
   created.

Disable staff markup on 'detached' blocks.

First pass at an API for XBlock testing.

Remove Pylint violations

Fix: Disable capa buttons to protect against race condition that may occur between two competing AJAX calls.

[SWAT-97] Logistration alignment and padding.

Staff markup was enabled on all block types in
https://github.com/edx/edx-platform/pull/10903

This works well inside the courseware, but it breaks layout of the course about
page, which is also an XModule, see:
https://github.com/edx/edx-platform/pull/10903#issuecomment-164266342

This commit disables staff markup/staff debug on all blocks except
blocks tagged with 'detached'. Detached blocks include course about
and info pages, static tabs.

Append block item only if it has path to root

Revert "Edited the text strings in the default template for web certificates to remove wording specific to edx.org"

Revert "Edited the text strings in the default template for web certificates to remove wording specific to edx.org"

Edited the text strings in the default template for web certificates to remove wording specific to edx.org

Code refactor

leanModal fixes

Disable certificate template creation for audit only courses

Remove the graphical slider tool

Edited the text strings in the default template for web certificates. Removed wording that applied specifically to edx.org. Reworded strings so that they will read correctly when the value of platform_name or cert_type requires the a or an article. Addresses DOC-254.

Add autogenerated migrations due to field default changes.

Fix auto set deadlines on publish

Bring DoneXBlock to production-level code

SOL-1528: Generate Certificates for whitelisted students even if they have existing (notpassing, error etc.) certificates 

MA-1051 - DiscussionAPI: Remove http errors from api.py

SOL-1418: display certificate generate history in reverse chronological order

Make pagination pattern sass globally available in LMS

LMS: addressing contrast issues on the Progress page

WIP for financial support tool.

Allows support staff or global staff to view a list of a learner's
enrollments, and change enrollment modes. We generate a
ManualEnrollmentAudit record for these enrollment changes in order to
track updates. Additionally, enrollment changes are handled through
the enrollment API, which handles bookkeeping such as granting refunds
where appropriate.

ECOM-2825

Release