Update forum new post form

Add help text to several fields, update visual styling, and make it
appear in the right-hand pane of the discussion tab instead of sliding
down from the top.

Co-authored-by: jsa <jsa@edx.org>
Co-authored-by: marco <marcotuts@gmail.com>
Handle question posts from comments service
Clean up dead forums front-end code
Unify forum thread views (and templates)
Distinguish forum thread types in rendering

Each thread's type is now included in its post details, and question
threads have marked answers shown immediately below the body of the
initial post.
Add endorsement info to marked answers in forum

Co-authored-by: jsa <jsa@edx.org>
Change endorsement UI for forum question threads

Responses to question threads can be "marked as answer" instead of
"endorsed".
Re-render forum responses when marked as answer
Add post type indicators to forum sidebar nav

The previously existing checkmark for threads with endorsed responses is
removed, as it is mostly redundant.
Add post type control to forum new post form

Also change server to pass post type through to the comments service.
Refactor a small amount of forum permission code
Allow authors of forum questions to mark answers

Co-authored-by: jsa <jsa@edx.org>
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>
Adding question and discussion categories for posts
Added endorsement and thread_type fields
New UI for discussions