LowBalance returns amount added, if any

Fix typo, add test

Merge branch 'overage-limit'

* overage-limit:
  auto_top_up_amount and monthly_overage_limit from CustomerRepo
  Move Bandwidth Tn remote operations to BandwidthTnRepo
  Use value_semantics to DRY up CustomerRepo
  Use NotFound exception for all customer not found cases

auto_top_up_amount and monthly_overage_limit from CustomerRepo

Move load of auto_top_up_amount to the repo, and load monthly_overage_limit in
the same operation.

No low balance if not registered

If cancelled or expired or not-yet-registered, don't auto top up or notify about
low balance.

Merge branch 'rubocop'

* rubocop:
  Additional fixes for rubocop 1.10.1
  Switch to rubocop 0.89.1

Switch to rubocop 0.89.1

This is the rubocop in new Debian stable

Bring in line with the key from billing_monthly_cronjob

Merge branch 'low-balance-auto-top-up'

* low-balance-auto-top-up:
  Some people have exactly 5 who don't need to be told
  On low balance, top-up or notify
  ExpiringLock helper

On low balance, top-up or notify

On start up, check for users with low balance and NOTIFY about them.  LISTEN for
such notifications and process by either sending a low-balance warning message
or else attempting an auto-top-up as configured.

Using NOTIFY/LISTEN because then we can NOTIFY after any INSERT that leaves the
balance too low (using a trigger).  Doing the sync on start-up in case we missed
a NOTIFY during any downtime.  Using the Redis lock to prevent spamming a
low-balance user in case of many restarts or if they have many small
transactions happen in one day.