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>
Merge pull request #4355 from edx/jsa/new-post-style

Update forum new post form