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.