Unittest jobs now fail if any test fails. Unittests on Bullseye no longer
fail due to jinja2 import errors. Linter jobs run faster.

Previously CHANGELOG had to be updated manually. This was done by downloading
the output of a CI job that used listed commits since the last release. This
made releasing a bit more complicated and led to uselsess release commits.

Now a script creates the full debian/changelog with all versions based on
git tags and git commits.

Patch-version is not pinned.

Fixes: #121
Fixes: #122
Supersedes: !93

Fixes #110.

This also adds CI tests for schema upgrading/downgrading on both SQLite and
MySQL/MariaDB.

Also fixed architecture of Debian packages.

Co-authored-by: nd <nd@cccv.de>

Add a few unit tests as well as integration tests that cover almost all views. Also fixed a lot of HTML validator errors.