Deal with unmaintained dependencies

[henrikhorluck]

There are several dependencies we use that have not been maintained for years. This is now causing some deprecation-warnings — eventually errors — due to the lack of updates. E.g. in #2431

There are several ways to solve this, examples being:

• (For smaller dependencies) write our own version, and maintain it ourselves
• Find alternative packages that do the same
• Remove it altogether

List of dependencies that should be dealt with:

• django-taggit-serializer
• django-chunks
• ...