Refs #795

When listing unused users, it was tried to show some kind of activity
measure. The activity bookmark timestamp was used, which is removed now.
As there doesn't seem to be an easy way to retrieve user activity, we
remove the corresponding output.

Refs #795

Results of the generator function were ordered by the removed activity
bookmark timestamp (unclear, why). We now order them by registration
date (oldest first).

Refs #795

In order to implement incremental updates, there are still some pieces
missing.

See #765

Previously the matrix ID of a recently removed user account was not
removed from its group's Matrix rooms, since the account object was already
gone, when the (delayed) task was executed.

Now all relevant information (Matrix ID of the user and the group rooms) are
retrieved *before* the delayed task function is called.

When a user clicks on a chat room link, we reply with a redirect to the
user's web client.
Somewhen around matrix-synapse v1.50 the path `/_matrix/client` stopped
returning a redirect to the site-local web client.
Thus we maintain a manual list of Matrix web clients until either
MSC2312 [1] is supported or a suitable redirect is published again
(see [2]).

See #793

[1] https://github.com/matrix-org/matrix-spec-proposals/pull/2312
[2] https://github.com/matrix-org/synapse/issues/14722

For unknown reasons it may happen, that the same key is resolved in
parallel (maybe due to asynchronous operations within the resolver
functions).
Thus, we do not rely on non-parallel execution anymore.

This reverts commit d1bc767d.

Sadly there is no way to *create* a PID file via systemd.
(the PID stored in a file would have been helpful for a monit check)

Such a location is helpful for publishing local branding assets.

The documentation still referred to the old Django-style configuration.

Now overrides like `core/_head.html` may be placed into this directory
without any manual configuration changes.

Fixup for commit 558bf00f.

Previously uploaded files, which got renamed due to the recent
filename normalization, were not accessible anymore.
They were not accessible, since the `/stadt/media/` path was still
handled directly by nginx (not Django).

Now the configured redirects are finally processed.

See #769

Fixup for commit 63953f88.

Migration `0005_normalize_filenames` created Redirects with wrong `old_path` content.
Thus redirects did not work (but they were not used, yet, anyway).

Closes: #796

Contributed by Robert.

We are still using the fine uWSGI daemon, but we do not use the
system-wide uWSGI setup anymore.