participant home screen api

Closes #91 (closed) Closes #93 (closed) Closes #94 (closed) Closes #95 (closed) Closes #96 (closed) Closes #101 (closed) Closes #102 (closed)

Proposed changes

• Established api connection for room service
• Adjust creator-room-list, room-creation/update(api ressources are still missing)
• Adjust room-joining, participant-room-list(api-ressources still missing)
• Added demo room joining

For testing

• Login as Creator => you should see the rooms which that user has created, you can join them
• Login as Participant => currently you can't see rooms the user has already joined yet

•    ""            => you should be able to join the ARSnova demo room by clicking the button