Refactor #airbrake_current_user

Fix for https://github.com/airbrake/airbrake/issues/313 based on the (excellent) work by @tonyarkles and his PR https://github.com/airbrake/airbrake/pull/331

- ensure we return the DB connection   
- split the DB interaction from populating the hash

The reason I prefer doing it this way is that it isolates the problematic spot into a single short method.
The middleware order is important and I could imagine all kinds of sites breaking should we change it. I'd avoid it if at all possible.