Add endpoint for accepting problem report submissions

These are anonymous submissions made through the "Is there anything wrong with this
page?" forms found on most GOV.UK pages, and come from the `feedback` frontend app.

These submissions are currently submitted through the `support` app but belong here, as part of the
frontend-backend split between `support` and `support-api`.
Plek now has a [`website_uri` method](https://github.com/alphagov/plek/blob/master/lib/plek.rb#L45) as well as `website_root`, which will make this code cleaner.