Closes #21 (closed) and fulfills its AC's, which were:

GIVEN a command channel with phone number +1-555-555-5555

AND and a channel with phone number +1-666-666-6666 and name #red-alert
AND a user alice, who is an admin of the channel
AND a user bob, who is new (not a member of any channel)
WHEN bob sends a message to +1-555-555-5555 that says add
THEN bob will be added to the subscribers of #red-alert
AND THEN bob will receive a signal message from +1-666... saying "you have been added to the #red-alert channel."

GIVEN the same conditions above

WHEN bob sends a message to 555... that does not start with the word join
THEN she receives an appropriate error message and usage instructions (eg: sorry: we did not understand that command. valid commands include...)

GIVEN the same conditions as above

WHEN bob sends a message to join a non-existent channel
THEN she should receive an error message saying that channel dos not exist (do NOT list channels that do exist)

GIVEN the same conditions as above

AND bob has successfully added themselves to the #red-alert channel
WHEN alice sends a message to +1-666..., both alice and bob will receive a message from that number

GIVEN the same conditions as above

WHEN alice sends a (second) "add" message to +1-666...
THEN she will receive a message from +1-666... saying "whoops! you already subscribed to #red-alert"
AND WHEN alice sends a message to the channel, bob does not see the message

Implementation Notes

Very busy right now! Rain check!... ;)

Resolve "Add self to group via signal message"