Remove self from a group via signal message

Value Statement

• AS AN overwhelmed activist
• I WANT to be able to remove myself from a group i am no longer intersted in
• SO THAT i don't burn out

Acceptance Criteria

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