Hide comments in marked answers in forum questions

A link to show the comments appears in their place.
Add filter menu to forum nav sidebar

The menu allows filtering to unread, unanswered, or flagged threads.
This includes passing through the appropriate query parameters to the
comments service.
Remove "flagged discussions" from forum topic menu

This feature is supplanted by the flagged option in the filter menu,
which can work within topics as well as across all topics.
Clear search alerts when a forum topic is selected
Update UI for forum actions

The actions are now consolidated in one location for each piece of
content. Primary actions (vote, follow, endorse, mark as answer) are
buttons, and secondary actions (pin, edit, delete, report, close) are in
a menu. This also includes improved front-end error handling for the
actions and significant test cleanup.

Co-authored-by: jsa <jsa@edx.org>
Co-authored-by: marco <marcotuts@gmail.com>
Co-authored-by: Frances Botsford <frances@edx.org>
Co-authored-by: Brian Talbot <btalbot@edx.org>