Rationalise environments

There are sensible defaults for test and production, so we no longer
need .env in order to run the tests - or indeed for anything.

Note that this *will* break Heroku deployment unless ENV vars are
changed.
Remove swathes of commented-out code

We can still refer to this via history - or, preferably, via the version
of Rails in use - but it makes it hard to see what's actually happening.