Misc. cleanup / fixes for Javascript UI:

- Update README
- Fix "Sign in" button.
- Fix "Test With Jenkins" buttons.
- Fix React legacy factory warning; see fb.me/react-legacyfactory
- Fix count in "All" tab
Add loading screen and caching / performance optimizations:

- Add loading screen that shows while PRs JSON is being fetched.
- Use Flask-Cache library to cache /search-open-prs JSON.
- Add cron job to keep the cache warm.
- Reduce the size of JSON reponse so it fits within the 1 megabyte
  memcache size limit.

Misc. operations / mainentance work:

- Bump version number to allow for rollbacks to the old version.
- Exclude some directories to speed up deployment.