Run full refresh periodically as a background task

I have chosen [clockwork](https://github.com/tomykaira/clockwork) which has nice DSL and could be easily adapted to work with any task runner.

This also needs [shoreman](https://github.com/statianzo/dokku-shoreman) as we have an additional process in the Procfile.
Style/LeadingCommentSpace: Missing space after #.