LEARNER-1923

TNL-6184

Fix read only support for team discussions

Fix error messages to use "posts" not "threads"

More small UI tweaks, padding and alignment

Add a11y tests for all inline discussion states

Improve discussion error messages

TNL-4756

TNL-4640

TNL-5163

TNL-4557

TNL-4622

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

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.

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>

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.