Tweak forum topic browse menu

The filter box will now remain fixed while the rest of the menu scrolls.
This also fixes a minor cosmetic bug in which the thread list would
overflow its container if the user opened the browse menu, scrolled the
page, and then closed the browse menu.
Remove sidebar class from forum nav

This will enable much cleanup of the relevant SASS rules.
Clean up forum nav Sass

Now that the sidebar class is no longer used, many rules no longer need
!important and can move out of _shame.scss.
Merge pull request #4318 from edx/gprice/forum-sidebar-debt

Reduce tech debt in forum sidebar