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
Revert "Merge pull request #276 from moneyadviceservice/advice_plan_command"

This reverts commit 7e1916519e5bebc12621b185a2399a71693fcd86, reversing
changes made to 29964acc478e923493d2818c69f666a2f33bb831.
Include and configure html_validation library
Test home page html validation
Validate a category page in both locales
Validate an article page in both locales