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.
fixup! Add filter menu to forum nav sidebar

Improve layout when cohort filter is present
fixup! Add filter menu to forum nav sidebar

Clear filters when a search is applied
fixup! Add filter menu to forum nav sidebar

Update text of options according to UX feedback, and add accessible
labels to selects.
fixup! Add filter menu to forum nav sidebar

Update jasmine spec fixture to match real template
fixup! Add filter menu to forum nav sidebar

Pass filter params through to CS from followed threads endpoint
fixup! Add filter menu to forum nav sidebar
Clear search alerts when a forum topic is selected
Merge pull request #4617 from edx/gprice/forum-filter-menu

Add filter menu to forum nav sidebar