Admin & Initial config page

[Sjors]

Currently everything is configured through ENV settings, which leads to quite a large AWS template form and some pain if a user makes a mistake there. It's better to have an admin page for things like SMTP settings, podcast settings, etc.

This should also reduce the dependence on rake tasks.

The page should have 2FA and probably run in an isolated process (a separate Rails app with its own server, if that doesn't consume too much memory?)