Refactor command list to use composition

Also rename buy credit to top up

Command to get to the credit card settings page

Rename buy-credit to buy credit

Merge branch 'conditionally-show-commands'

* conditionally-show-commands:
  Only show buy credit option if customer has a plan and a credit card
  Show commands conditionally (plus add voicemail record command)

Only show buy credit option if customer has a plan and a credit card

Show commands conditionally (plus add voicemail record command)

Refactor to a factory that allows commands to be shown conditionally, so that
the voicemail record command can only be shown if the customer has a forwarding
target set up.