Raise TypeError if the request is not in the proper format for UcamWebAuth, so that django tries other authentication backends

It's no longer required.  Arguably, we should run some tests with it
set to make sure it still works.

It's excessively picky, and in particular rejects single-component host
names.  These are permitted by the URI spec, and are generated by
django.test.RequestFactory.  In any case, the next thing we do with the
returned URL is to compare it with one we've generated internally,
which should catch any genuinely invalid URLs.

This is a two-character fix with a 17-line test case.

It can now be automatically generated from an incoming request and
Django's urlconf.  Django handles validation of the host name by checking
against ALLOWED_HOSTS.

This causes relatively deep code changes because it requires passing the
current HttpRequest a lot deeper into the call stack than it used to go.