Add group creation and user deletion endpoints

[the4t4]

Added:

[POST] groups/create

• Request body: {groupname, timetable?}

[DELETE] users/

• Request body: { users: [{userid}] }

The delete endpoint is not complete as it does not remove the user from keycloak yet. Its best if @Mohido can complete that.

[Mohido]

I will add the Keycloak deletion user deletion later on today and then we can merge this branch to the development.