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>