This is needed because jinja2 had an update which caused the portal to
not work at all:
```
ImportError: cannot import name 'escape' from 'jinja2'
```
Quart needed updating for that.
This update required a lot of typefixes. Apparently, the "canned"
responses (like redirect) are now plain werkzeug responses, while
quart.Response does not inherit from werkzeug.Response (otherwise, we
could've changed the type annotations to werkzeug.Response everywhere,
but that doesn't work because a quart.Response is not a
werkzeug.Response).
P.S.: This time, I *did* check that avatar uploads don't break (see
b007afc).
This allows future App versions to also work with the invite page
without having to screen scrape the content.
Fixes#56 (at least for the portal side of things).
That seems to be some Quart-internal redirect which isn’t executed
correctly (probably due to our makeshift error handlers). So I
make this a proper redirect instead.
