The LMS should serve favicon.ico also when a theme is enabled.
Merge pull request #3769 from shindere/favicon-fixup

Improves the way LMS serves /favicon.ico
Move video descriptor instantiation in tests to single place.
Merge pull request #4086 from edx/alex/refactor_video_test_2

Move video descriptor instantiation in tests to single place.
Display names, help text, and "deprecated" for Advanced Settings.

STUD-302, STUD-303
Help string edits

Removed not supported notes on deprecated settings, as deprecation is
already shown. Edited xblock mixing help.
Test updates.