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.
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
Merge pull request #3597 from caesar2164/giulio/help-modal-features

Changes for Help Modal to work on Stanford Site
Bok-Choy Video Tests Batch6