[v3 / Feature Req.] add some helpful permissions/settings

[PhilippKilian]

Some permissions would be helpful to prevent unwanted user behavior...

• user of a role can...
  • pre-uploaded presentation slide for a room (currently only possible via global settings for all users or no user)
  • add an avatar if allowed (prevent unwanted type of pictures as avatar)
  • delete their account if allowed (as option that only administrator can delete them)
  • cannot delete their default room (same behavior as in Greenlight v2)
• room codes can be edited manually