Ubiquitous session favoriting

[pcantrell]

My most wished for feature: ajaxy buttons throughout the UI (session lists and schedule) for voting/unvoting! Now you can see your favorites on the schedule. Woohoo!

This is not quite done, but I have to run off and be a dad for a while. Anybody want to pick it up? Things to do right now:

• Cross browser testing, testing, testing (I've done almost none)
• ~What is up with those two failing specs? I'm doing something dumb.~

Things to do in the future, but not in this PR:

• Better testing
• Revisit the “Yes! I’d like to attend” behavior, which still doesn't have an unvote ability … but is complicated because of the embedded login form.

Notes:

• I also revamped how some things get cached, hopefully for the better. Entire schedule view is now served from cache — but uses session update times, so no more manual cache busting if you tweak a session!
• I wrote the session favorite so that it does not break caching. There is one cached version of the schedule for everyone; a separate call from JS fetches your voted session IDs and highlights the appropriate sessions.

[coveralls]

Changes Unknown when pulling 6902cc03c6bbd1771372a34e334af1673018820e on ubiquitous-session-favoriting into on master.