As a user, I want to be able to create groups and invite other users to join these groups.

[lguebeli]

Acceptance Criteria:

• Users which create a group are automatically the administrator of this group

• Group administrators can set a group name and group description.

• Users can invite other users to join the group by sending a group specific code.

• [x] sopra-fs24-group-29/rapidhabit-server#61

• [x] sopra-fs24-group-29/rapidhabit-server#62

• [x] sopra-fs24-group-29/rapidhabit-server#63

• [x] sopra-fs24-group-29/rapidhabit-server#64

• [x] sopra-fs24-group-29/rapidhabit-client#14

• [x] sopra-fs24-group-29/rapidhabit-client#15

• [x] sopra-fs24-group-29/rapidhabit-client#16