Closes: #253

Previously only a few documentation files were examined.

Previously it was not possible to import data, since the `post_save`
signal handlers created new entities, which were supposed to be imported
a bit later.

Now it is possible to migrate data even if different database engines
are involved:
* export:
    ```shell
    lohroctl dumpdata \
        --natural-primary \
        --natural-foreign \
        --exclude sessions \
        --exclude admin.logentry \
        --indent 4 \
        --all \
        > lohrothek-dump-$(data +%Y%m%d).json
    ```
* import: `python3 manage.py loaddata --format=json - <dump.json`

Thanks, codespell!

We should rely on Django for generating timestamps (with a timezone).

Only very few content-related changes are included (e.g. conversions
from list/filter to list comprehension).

Hidden behind a bare except-clause there was a type mishandling.
The problem was covered by falling back to the current time in case of
errors.
A test was added for detecting this issue.

This reduces the noise caused by code or template changes.

It possible again to add Audiofiles to RadioReports in detail view.

The ContentTag and the MinimalTagSerializer are now including the
`id` of the tag. The corresponding test is now also fixed using
this.

We will apply these styles later.

Since commit 675bfa1e there is no view containing the
"create-audiofile" url.  Thus we cannot test the proper creating of an
audio file.  The test is skipped for now.

In case the redirect is really necessary, we should use
`client.get(..., follow=True)` instead.

For now all tests require a logged in user, but this may change in the
future (for permission tests).
Thus we should separate it now instead of requiring more effort later.

The Mixin classes are not supposed to work stand-alone - they always
rely on a real base class being inherited independently.

The tests are run as usual - but in addition the code coverage is
determined, as well.

This allows contributors to run the pipeline in their forked repositories.

Fixup for commit 675bfa1e.

Fixup for commit 7ae4c2b9.

Add new foreign key to Audiofile model. This gives the possibility
to add recordings to an episode.