api: Automatically give permissions when user is assigned

[mathiashoeld]

When a user is assigned to a workflow item, the permissions for viewing, closing, etc. should be granted automatically.

[Stezido]

Since we want the api to prevent executing user intents with unexpected behaviour like granting permissions when assigning users, we should handle this on UI-level. See #23

[mathiashoeld]

idea: the process should not be totally automatic, but rather there should be an option (e.g. checkbox) that enables permissions for the parent project/subproject