Rendering model states when running migrate in Django 1.8 consumes
on the order of 4x-6x more memory than <1.8.

References:
* https://code.djangoproject.com/ticket/24745
* https://code.djangoproject.com/ticket/24743

This is causing failures in Jenkins as the workers run out of memory
and either processes cannot fork or the Out Of Memory Killer
kills a process that is running tests.

Disabling migrations until we do either or both of these:
* Upgrade to Django 1.9 and turn them back on. See EV-92 "Discovery: Upgrade edx-platform to Django 1.9"
* Address TE-1310 "Unit tests should be able to run without migrations"

Adds a context-sensitive link to learner documentation from the LMS. The base URL defaults to Open edX learner doc and can be overridden with the base URL for edx.org learner doc.  Also relabels the tab for support from help to support.

Fixed import failures that were preventing the platform API documentation from building. The problem is that the doc building environment doesn't need to install all the Python modules that the platform software does, but when Sphinx tries to import select modules so that it can use autoclass to publish docstrings it tries to import all the modules that are imported by those modules, recursively. I mocked the missing modules.

Meta.

TNL-4906 Subsections now use radio buttons, to allow for "hide after due" as a
visibility option. Also, all tabs have been consolidated to "Basic" and
"Advanced", and visibility options have moved there.

Documentation links are updated to assist course authors with the new
visibility options. Tests have also been updated, and the changes suggested
in TNL-4951 are included.

Mocked some Python modules when running a Sphinx documentation publishing build. Updated an autoclass class name that changed."

Adding the -n option to the Sphinx build invocation so that WARNINGs will be treated as ERRORs. This will cause builds to fail if Sphinx cannot import modules for its autoclass directives. Currently, they fail silently and leave gaps in the published HTML where docstring content should have been.

Adjusted the list of mock modules for the platform API documentation build. This prevents build failures when Sphinx cannot import modules. Addresses https://openedx.atlassian.net/browse/DOC-2638.

It's just as relevant to unit tests and I wish I had seen it earlier.

Stress the transient nature of this debugging step.

This reverts commit 8402be3b.

Ran these suckers through ImageOptim, which crunched 'em down real nice.

 common/test/data/xml_dag/static/just_a_test.jpg    | Bin 547 -> 517 bytes
 .../test_microsite/images/background-image.jpg     | Bin 160660 -> 160625 bytes
 .../test_microsite/images/header-logo.png          | Bin 1215 -> 1212 bytes
 docs/en_us/architecture/locator_ubiquity.jpg       | Bin 51876 -> 47455 bytes
 docs/en_us/architecture/presplit.jpg               | Bin 43129 -> 38944 bytes
 docs/en_us/architecture/studio mapping.jpg         | Bin 48247 -> 43786 bytes
 .../enrollment_api/source/_static/homepage-bg.jpg  | Bin 160660 -> 160625 bytes
 docs/en_us/internal/test_pyramid.png               | Bin 24698 -> 17598 bytes
 .../platform_api/source/_static/homepage-bg.jpg    | Bin 160660 -> 160625 bytes
 .../shared/images/Announcement_subscriptions.png   | Bin 43413 -> 15474 bytes
 lms/static/certificates/images/backpack-logo.png   | Bin 2107 -> 1112 bytes
 lms/static/certificates/images/backpack-ui.png     | Bin 76558 -> 54810 bytes
 lms/static/certificates/images/bg-verified.png     | Bin 25152 -> 5169 bytes
 lms/static/certificates/images/demo-sig1.png       | Bin 3852 -> 2099 bytes
 lms/static/certificates/images/demo-sig2.png       | Bin 3695 -> 1990 bytes
 lms/static/certificates/images/demo-sig3.png       | Bin 7315 -> 3721 bytes
 .../certificates/images/demo-user-profile.png      | Bin 61749 -> 40719 bytes
 lms/static/certificates/images/ico-honor.png       | Bin 59443 -> 35743 bytes
 .../certificates/images/ico-mozillaopenbadges.png  | Bin 3638 -> 1870 bytes
 lms/static/certificates/images/ico-verified.png    | Bin 59571 -> 35713 bytes
 lms/static/images/app/google_play_badge_45.png     | Bin 8913 -> 4548 bytes
 lms/static/images/bg-footer-divider.jpg            | Bin 416 -> 390 bytes
 lms/static/images/bg-texture.png                   | Bin 10028 -> 2092 bytes
 lms/static/images/bulk_email/FacebookIcon.png      | Bin 550 -> 333 bytes
 lms/static/images/bulk_email/GooglePlusIcon.png    | Bin 1286 -> 679 bytes
 25 files changed, 0 insertions(+), 0 deletions(-)

Fix doc build failures that were caused by Python module import failures. Also fix broken cross references. https://openedx.atlassian.net/browse/DOC-2478