Add preliminary search interface

TODO:
- Test cancellation properly
- Get rid of localstyles.css, integrate into Extension:MobileApp
- Finalize design of search with designers
- Implement 'Quick Return' pattern when scrolling

Wikidata based search is blocked on a wbsearchentities patch
being merged, so just using opensearch API in the meantime

Change-Id: I9f9c4f127b23293666a5ccfa7b738fd6ab5c9f60