Recover from error loading forum thread list

When a user attempts to load more threads in the forum navigation
sidebar, reset the state of the world so the user can retry, and alert
the user appropriately.
Add focus trap on forum navigation thread loading

For accessibility purposes, it is bad to allow a user to initiate
loading of additional threads in the navigation sidebar and then shift
focus away from the sidebar, only to have focus snap back when the
additional threads are loaded. Now, we trap focus on the loading element
as recommended by our accessibility consultant.

JIRA: FOR-238
Merge pull request #1707 from edx/gprice/forum-thread-list-focus-trap

Add error recovery and focus trap to thread loading in forum sidebar