As a user, I want to be able to join a group so that I can participate in collaborative habit tracking.

[lguebeli]

Acceptance Criteria:

• Users can join a group by using their invitation code and enter it on their dashboard.

• Users receive a confirmation of success after joining a new group.

• Users can see the success of the other users in the group after joining a new group

• [x] sopra-fs24-group-29/rapidhabit-server#23

• [x] sopra-fs24-group-29/rapidhabit-client#37

• [x] sopra-fs24-group-29/rapidhabit-client#35

• [x] sopra-fs24-group-29/rapidhabit-client#20

• [x] sopra-fs24-group-29/rapidhabit-client#21

• [x] sopra-fs24-group-29/rapidhabit-client#22