* 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>

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

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.

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>

This will enable much cleanup of the relevant SASS rules.

The home button is removed, the search box is always displayed, and many
visual adjustments are made, along with a large amount of refactoring.

Originally reviewed in #4211

Thread entries now show an activity count that includes the original
post, and only one of the vote count and activity count is displayed
based on the user's selected sort. The sorting and filtering options now
both use a select and are somewhat more verbose, and the visual styling
of the sort/filter bar is updated.

Originally reviewed in #4165 with a bugfix in #4211

A thread will be labeled if it is pinned, if the user is following it,
or if it was authored by a staff member or community TA. This also
slightly changes the color used for community TA labels.

Originally reviewed in #4072

Use larger and lighter title font, decrease padding, and use lighter
solid backgrounds instead of gradients. Also, refactor SASS code to
conform better to our new style, including using more specific class
names.

Originally reviewed in #4072

Co-authored-by: Brian Talbot <btalbot@edx.org>

JIRA: FOR-591

visual styling adjustments to discussion listing metadata, simplifying the visual representation of votes, increasing the line height of each post while also increasing the size of the icons for following and comments. visual styling adjustments also included for the discussion itself, pushing comments to be full width within a response, and small typography adjustments to the post count metadata and the add response button

1. Add space between threads
2. Change Show/Hide to Expand/Collapse and visually separate it
3. Limit a thread to 700px high and scroll if longer

Co-authored-by: marco <marcotuts@gmail.com>

Students no longer see a tooltip nor any button affordance (i.e. cursor
and color change on hover/focus) on the pinning feature. For moderators,
the tooltip is now correct when a thread is pinned, the button is
accessible for keyboard-only users and screen readers, and an error
dialog now appears if an error occurs in attempting to pin/unpin.

JIRA: FOR-192

Users will first be shown the first 25 responses of a viewed thread and
then have the option of loading 100 more at a time until all responses
have been loaded. This mitigates the performance impact of very large
threads by avoiding the need to render and transfer over the network
all content of a large thread at once. It will also allow students with
slower browsers to interact with large threads even if they cannot load
all of the responses.

- Clarify checkbox description wording
- Wrap checkbox and envelope icon in div to make it stand out
- Added styling for wrapper div
- Wrap label element around div to allow clicking the envelope to activate the checkbox
- Indented HTML and fixed if wrapping issue

JIRA: FOR-373

This includes removing the "PREVIEW" label for the preview pane from CSS
and adding it to the generated DOM.

This button has been broken for at least a year, so the code has
suffered from bit rot and should be reimplmemented if the feature is
necessary in the short term (which is unlikely since it has been broken
for so long).

- Doubled up :hover and :focus styling to improve a11y
- Increase contrast of certain UI elements for improved a11y
- Added some image alt text for a11y
- Changed video caption styling to blue and made them underline on hover and added a skip link before video for screenreaders.

Fixes Bugs:
- LMS-1336

The icons and text for pinning and flagging ("Report Misuse") had
contrast ratios that did not meet the minimum WCAG requirement of 4.5.
This fixes FOR-200.

Previously, colors changed to afford interaction only on hover; now, all
elements with a hover color change behave the same way on focus.