This change reworks the way themes are built in Gerrit. It:
- Moves themes (so far, just app-theme and dark-theme) to a subdirectory
of app/styles for organizational purposes.
- Explicitly includes this directory in the PolyGerrit bundling process.
- Enables gr-app to (lazily) load dark-theme.html if it recognizes the
'dark-theme' key in localStorage.
Upon merge/deployment of this change, users may dogfood the dark theme
by setting the key 'dark-theme' in localStorage.
Change-Id: Id7449e0bdd4f12a9f1e44de6b83003fdb253a0b0
61aff034a0