WIP: Get sidebar code running in a panel in new versions of Firefox


Supercedes PR #354.  All reviewing comments addressed.  Two things remain to do before landing:


* debug onUserSignoutRequested by forcing XXX comment to be true
 and debugging further (figure out how to write a test for the ultimate bug and maybe this handling!) 

* reproduce debug whitespace around just name


Gets the sidebar code working in panels, a la:

https://trello.com/c/lX521lFu/304-2-as-a-user-i-can-use-talkilla-in-a-panel-rather-than-a-sidebar

This should be ready for review.

Note that a few things have been left for upcoming cards:

* work correctly in a version of Firefox different from the one the user installed it in
* serve Windows-specific icons for windows