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