Allow cliend-side caching

Add Expires and Cache-Control headers to API responses. This allows
browsers to cache them and avoid extra round-trips for data. Also use
plain JSON type in UI to allow browser to use cache. For widget to work
added "Access-Control-Allow-Origin: *" header to all API requests. Note
that althogh this will make cross-origin requests work, it's still not
enough to make browser cache such response, so widget won't benefit from
client-side caching yet.

Change-Id: I49eeffccaf79fe3417bd941590b5dd239ddc433f
Merge "Allow cliend-side caching"