Stub AdvicePlans::StartTaskCommand

We are sharing sessions with the desktop website. The Advice Plans
engine stores command objects in the session. Rails attempts to fetch
the whole session graph causing a `uninitialized constant AdvicePlans`
error.

This commit introduces a stub for that object that allows us to continue
to share the session.

See https://github.com/moneyadviceservice/advice_plans/blob/master/app/controllers/advice_plans/tasks_controller.rb#L35
Stub all constants under the AdvicePlans module

`AdvicePlans::StartTaskCommand` takes other objects within the
AdvicePlans module as arguments. This stubs out the whole AdvicePlans
module.
Merge pull request #276 from moneyadviceservice/advice_plan_command

Stub AdvicePlans::StartTaskCommand