BLD-722: CodeResponse i18n.
BLD-714: CustomResponse i18n.
Changes for Help Modal to work on Stanford Site

- moved zendesk endpoint to new condition
- some help modal styling
IGNORABLE_404_ENDS is wrong and unused

This setting is only used if SEND_BROKEN_LINK_EMAILS is true, which it
is not, and it is deprecated, and this value for it is wrong, since it
is used as an iterable.
Convert lettuce video tests specifically using youtube stub
Merge pull request #3492 from edx/ammar/bok-choy-youtube-stub-video-tests

Convert lettuce video tests specifically using youtube stub
Fixed cheatsheet for open ended editor.
STUD-1583
Merge pull request #3510 from edx/waheed/stud1583-toggle-cheatsheet-not-working

Fixed cheatsheet for open ended editor.
Merge pull request #3608 from edx/anton/i18n-code-response

CodeResponse i18n.
Merge pull request #3586 from edx/anton/i18n-custom-response

CustomResponse i18n.
Describe date format for close/reopen discussions
Merge pull request #3568 from edx/ahodges/documentation/DOC258

Ahodges/documentation/doc258
Merge pull request #3628 from edx/ned/fix-ignorable-404-ends

IGNORABLE_404_ENDS is wrong and unused
Add peer- and self- only information
Merge pull request #3597 from caesar2164/giulio/help-modal-features

Changes for Help Modal to work on Stanford Site
Fixed typo

doc-308
Merge pull request #3635 from edx/sylvia/docs/TIM-464

Add peer- and self- only information
Merge pull request #3637 from edx/markhoeber/documentation/doc-308

Fixed typo