Resolve "admin can add/remove other admins via signal message" (#27)
Closes #27 (closed) and fulfills its AC's, which were:
GIVEN alice is admin of channel foo with number +15555555555
- AND bob has phone number +14444444444
- WHEN alice texts "ADD ADMIN +14444444444" to +15555555555
- THEN bob will be made an admin of foo (he will receive all messages on foo and be able to broadcast messages to all subscribers)
- AND THEN alice will receive a confirmation message
GIVEN the above
- AND bob is already an admin of foo
- WHEN alice texts "REMOVE ADMIN +14444444444" to +15555555555
- THEN bob will be made no longer an admin of foo
- AND alice will receive a confirmation message
GIVEN the above
- AND bob is already an admin of foo
- WHEN alice texts "ADD ADMIN +14444444444" to +15555555555
- THEN no new administration record will be created
- AND alice will still receive a confirmation message
GIVEN the above
- WHEN alice texts "REMOVE ADMIN
<ome incorrectly formatted number>" - THEN she will receive a helpful error message to correct the formatting
GIVEN the above
- WHEN alice texts "REMOVE ADMIN
<number of someone who is not an admin>" - THEN she will receive a helpful error message letting her know the person is not an admin