support multiple channels

Value Statement

AS A maintainer of signal-blaster

• I WANT to be able to add new channels to the same instance of the app while it is running
• SO THAT I can test the app on a different channel than the one being used in the field
• AND SO THAT I can quickly add new groups/users without having to redeploy the app

Acceptance Criteria

GIVEN a set of channels (each with a unique phone number and list of admins) stored in the DB

• WHEN the application starts up
• THEN it will start a separate process for each channel each listening for Signal messages to that channel's phone number and relaying messages from its admins to its members

GIVEN the above

• WHEN a maintainer submits a properly formatted, authenticated payload to /channels/new
• THEN a new channel (and appropriate join records) will be created in the database
• AND THEN a new process will be started to listen for messages on that channel

Implementation Details

• for API authentication: check for random 40-digit hex string in payload (simple token) <-- ask homies if this is good enough
• payload should look roughly like:

{
  "authToken": "0xED7E8B209A4CC8889A7A2C5F561389EE46F53562",
  "channel": "+15555555555",
  "admins": [
    "+12222222222",
    "+13333333333"
  ] 
}