[cleanup] grab-bag of misc cleanup tasks

This MR fixes some stray TODOs, borked unit tests and conditions that are less-than-friendly to friendos-of-friendos trying to run their own signalboost instances.

In particular it:

• removes some stale TODOs

• fixes some broken unit tests from !184 (merged)

• modifies ansible/provision.yml to allow 3 failed auth attempts for ssh access to prod instead of 2

• un-hardcodes ngrok subdomain in registrar configs (provide it as env var!)

  • this makes it so friendos-of-friendos can't register numbers with twilio!

• prevents crash on rate-limit error if there is no signup channel (since a signup channel might only exist on team-friendo's signalboost)

  • context: we want to notify admins of the signup channel if there is a rate limit, but some signalboost instances that don't belong to team-friendo might not have signup channels!
  • cause: we try to map over the memberships field of a channel that doesn't exist (uh-oh! can't call .map on null! crash!)
  • fix: if there is no signup channel, just return early and don't notify, prevent calling the code that crashed

• avoids reparsing phonenum from ADD/INVITE if reseting expiry

  • context: it's sorta costly to reparse a command for a valid phone number, and unnecessary given we know it contained a valid phone number!
  • instead: pass along the payload from every command into its command result, and just pluck the payload off of the ADD/INVITE commandResults in dispatcher.messenger.setExpiryTimeForNewUsers