Most of Themes V2 + a bunch of other things

This commit includes most of Themes V2, but until I had it functional I
kept other changes local.

While working on that I also:
Changed OAuth in widgets to only init when needed for faster page loads
Added help text to the Stocks widget
Fixed a parsing bug in the Stocks widget
Redesigned the Store
Fixed a bug in the Spectrum plugin
Added lazy-loading to the Store and Apps dropdown
Added support for custom CSS

I'm sure there's more but Github is freezing and won't show the diff,
and I can't remember what else I did.