Clients disconnecting before we finish processing the request happens from time
to time. We don't need to yell about it

This ended up being a bit more invasive than I'd hoped for (not helped by
generic_worker duplicating some of the code from homeserver), but hopefully
it's an improvement.

The idea is that, rather than storing unstructured `dict`s in the config for
the listener configurations, we instead parse it into a structured
`ListenerConfig` object.

Broke in #7534.

Allow REST endpoint implemnentations to raise a RedirectException, which will
redirect the user's browser to a given location.

Python will return a tuple whether there are parentheses around the returned values or not.

I'm just sick of my editor complaining about this all over the place :)

* Fix servlet metric names

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Remove redundant check

* Cover all return paths