Resolve "Remove self from a group via signal message"

Closes #22 (closed) and fulfils its Acceptance Criteria, which were:

GIVEN a group #red-alert on phone number +1-666-666-6666 with admin alice and member bob

• WHEN bob sends a message saying "leave" to +1-666-666-6666
• THEN bob will be removed from channel's subscribers
• AND THEN bob will receive a message saying "you have been removed from #red-alert.
• AND WHEN alice sends a message to #red-alert, bob will not get it

Note that for speed's sake I omitted to mention the channel name in responses to both JOIN and LEAVE commands. (We can add that later if users want it.)

Implementation Notes:

• add channelRepository#removeSubscriber
• add LEAVE command to command service (delegates to commandService#maybeRemove)
• replace ADD command with JOIN command
• introduce performOpIfChannelExists helper to DRY up channelRepository functions