Add RBAC

[ztefanie]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
As a developer it is annoying to add new functionality that needs some form of validation if a user is allowed to do something.

Describe the solution you'd like
Add RBAC (role based access control)
Add to the data data model with the entities "Role" and "Permission"

Role: Defines the role of a user, e.g., Admin, region-admin, etc.
Permission: Represents specific actions, such as is-allowed-to-create-regions, is-allowed-to-see-statistics, etc.
Add a many-to-many relationship between "Roles" and "Permissions," as well as a many-to-one relationship between "Administrators" and "Role."

Adjust the current checks, so they check for permissions and not roles

Describe alternatives you've considered
Leave everything as it is.

Testing
Check if every user of different roles is still able to do what she/he is supposed to to and nothing else.