Improve community blogs

[Kirby]

Right now the UI for creating community blogs looks like this:

Entering User / Usergroup IDs seems unintuitive, especially as users normally won't even be able to know usergroup IDs.

It also gives them quite a lot of power.

I therefore suggest the following changes:

1. Add integer usergroup permissions to limit the max. amount of allowed contributor users and usergroups
2. Add a bool usergroup permission to restrict which users can be added as contributors
3. Add a multi-seletion option to configure which usergroups can be added as contributors, if none are selected the option to add usergroups should not be available
4. Use token input with autocomplete to enter usernames and usergroup titles instead of IDs