Change room and groups

• remove status information from room object (e.g. settings, closed)
• remove contentGroups from room object
• remove state from content object
• new endpoint for roomstatus information:
  • get contentgroup names and count for content (both visible and not visible for students)
  • get other status info like how many students are in the room
  • manipulate state as owner/moderator
• new endpoint for content groups (via group name)
  • get the sorted content ids with their content state info (e.g. visible for students, solution visible)
• WebSocket (STOMP messages via room topic) events for room / content state changes

additional:

• removes the autosort because with FE sorting in the content groups it's really not a good idea to split responsibility over sorting
• FE can paginage the contents in the content groups pretty easily
• FE can easily implement a good caching mechanism
• a student will never be able to see content that's not inside a group
• it takes the user 1 request to get every static information about the room that can easily be cached
• it takes an additional request to get all status information and "the bottom half" of the room that's more dynamic

p.s.: this issue may need to be documented better