Fetch remote styles once a day and cache them

- Fetches styles from bits betalabs and caches them
- Uses downloaded styles if they are more recent than packaged ones
- Fetches styles once a day (for now)

TODO:
- Do not fetch new CSS if it hasn't been changed
- Include sitecss (Mediawiki:Mobile.css) if available, and
  vary CSS based on the site the current page is for

Change-Id: I0b11420d75959ac546746daa9a21705598c9ee7d
Merge "Fetch remote styles once a day and cache them"