Refactor Invitations

- Make the requester_id mandatory
- Add invitation detail view
- Implement invitation list view
- Make the create and detail view generic
- Make the invitation overview list a reusable component
- Add check for changes in requester or requested id to form
- Update Tests
  - Add InvitationTestCase with generic send invitation and send action
    methods
  - Update HabitatInvitationTestCase to use InvitationTestCase