Adjustment to suspended users

## Feature Request

**Is your feature request related to a problem? Please describe.**
At the moment, when a user is suspended they can still perform actions on the forum such as changing their avatar. This could potentially lead to avatars or signatures that are derogatory in nature or otherwise break the community rules.

**Describe the solution you'd like**
I propose that;
- we adjust suspended users to move them to a specific (customisable) group (such as “suspended” or “banned”)
- This group has adjustable permissions to select what can and can’t be done.

This approach would allow extensibility such as allowing users to post to help tags or byobu, so that they can chat with the team or dispute a ban without it spilling out to the forums.
I’d further propose: removing the user avatar, and denying the ability to set a new one (obviously adjustable by permissions)

No files, tests, or entry points are named. Start by locating how suspended users, groups, and permissions are currently represented, then clarify the intended customizable restrictions and avatar behavior before implementation; done means suspended-user actions follow the configured group permissions.

Written by the indexing model from the issue text.