Refactor web-register to use lib/command

Move CustomerFwd behind Customer

All the previously-lazy BackendSgx data is now either all loaded or all not
loaded by swapping the sgx_repo used by your CustomerRepo instance.  When not
loaded the fields are filled with bottom values that explode when used.  When
loaded the values are present in RAM and not promises at all.  Most code paths
do not need any of the data, a few need most of it, so this seems like a good
trade-off.  Most code using this object will simply never touch those fields or
care about how they are loaded, etc.

Of course, most of this data isn't even SGX related and should move out of here,
but that would take a data model refactor/migration on the catapult_* schema.

Merge branch 'command-object'

* command-object:
  ErrorToSend => FinalStanza
  Since Command#finish causes an error, a then off the end won't work
  Refactor commands to have Command and Command::Execution objects

Refactor commands to have Command and Command::Execution objects

Brings the common elements of all commands together, and threads the most useful
state (such as ability to reply) through automatically using the new EMPromise
fiber trampoline.

Factor out CustomerRepo

Using the Repository pattern to encapsulate the fetch and create operations on
the persistence layer for a domain object.  These were not really factories in
the classic sense, but rather "fetch from persisitence layer" methods, and so
they now have a home.

Alt top up command

To show mailing address, eTransfer address, Bitcoin addresses, and allow
generating a new Bitcoin address.

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.