It is more suited for `process_request`, since it should stop
execution of the request if the domain is invalid. This code was
likely added as a process_response (in ea39fb2556) because there was
already a process_response at the time (added 7e786d5426, and no
longer necessary since dce6b4a40f).
It quiets an unnecessary warning when logging in at a non-existent
realm.
This stops performing unnecessary work when we are going to throw it
away and return a 404. The edge case to this is if the request
_creates_ a realm, and is made using the URL of the new realm; this
change would prevent the request before it occurs. While this does
arise in tests, the tests do not reflect reality -- real requests to
/accounts/register/ are made via POST to the same (default) realm,
redirected there from `confirm-preregistrationuser`. The tests are
adjusted to reflect real behavior.
Tweaked by tabbott to add a block comment in HostDomainMiddleware.