Move find_best to app

Questions:
 - finding an app for builds#index, do we need an API key? What's the use case for builds#index anyway? (can't remember)
 - I removed the test cases [here](https://github.com/tedconf/front_end_builds/pull/14/files#diff-7ca47115234d4001447d5896c72477efL82). Didn't fully understand them, but I think we don't need them