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

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

Includes info about the serversonly/testsonly pattern, as well
as multiprocessing.

TNL-3137

TNL-3137

https://github.com/edx/edx-platform/pull/9345

TNL-3445