Discussion forum styling cleanup to correct current visual styling issues in addition to cleaning up styles in preparation  for adding post images

This is a squash of 38 commits ending with
5b080f979d692804452400ac5bed9b17c50b001e

updated inline discussion styling to account for lack of expand / show message (which was being relied on for layout) and also removed issues of generic pagination styling impacting the older inline discussion pagination pattern with a class renaming

* updating vendor files
* updating class syntax (to new FA-based classes) for all UI elements
* correcting broken tests

SASS compilation variable conventions update, focusing on basic colors, baseline variable usage for padding and margins, ui-depth extends, along with other small fixes. The intent with this is to not have any user-facing impact, and instead move us closer toward having files that use our newer sass conventions.

spinner.gif and spinner-on-grey.gif are the same file.
Explicitly give the LMS a spinner.gif file.

OPEN-328

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>

The menu allows filtering to unread, unanswered, or flagged threads.
This includes passing through the appropriate query parameters to the
comments service.

A link to show the comments appears in their place.

Co-authored-by: jsa <jsa@edx.org>

Also change server to pass post type through to the comments service.

The previously existing checkmark for threads with endorsed responses is
removed, as it is mostly redundant.

Responses to question threads can be "marked as answer" instead of
"endorsed".

Co-authored-by: jsa <jsa@edx.org>

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.