automated backup (#40 part 1)

Context

first leg of epic described in #40 (closed). builds off of work submitted in !158 (closed)

Behavior

• every night run a cron job runs that:
  • makes a copy of the signal_data volume and pg_dumps the db
  • writes encrypted versions of both to the filesystem (encrypted to 2 maintainer keys)
  • scps those files to a backup server
  • destroys old backups on the backup server

Implementation Note

all files necessary to do this (cron tab file, scripts, ssh keys) should live in the repo (so a user logged into prod can set things up by pulling repo and cd'ing into correct dir)