The user management concept should be improved and integrated in the API

[schettn]

Describe the feature or change you'd like
Currently the user management depends on the wagtail CMS. Therefore it is not possible to manage user
and their project affiliations via the API.

I suggest implementing following features:

• Create a ohrwurm maintainer along with the cisco user. This user must not be superuser and has therefore no
  access to wagtail. The ohrwurm maintainer is used to create and manage user/project affiliations.
• API mutations for managing users.
• A user should be able to change his password via the API.
• Create permission groups for ohrwurm maintainers and members.