Resolve https://github.com/medic/medic-os/issues/22.

These nginx rewrites make dashboard's `_couch` facility available at `/_couch`,
and redirects everything under `/_utils` to `/_couch/_utils`. The explicit
references to index.html prevent a redirect loop from forming.

Tested pretty thoroughly on the latest development build of Medic OS. Hopefully
I didn't miss anything.