Embed anon feedback explore form in a panel

This is done in preparation to adding more forms to the explore page.
Explore page: add guidance

This informs the Feedex users about where the feedback originates.
More appropriate naming for Explore

* Explore => ExploreByUrl (to allow addition of other explore query classes)
* Explore#by_url => ExploreByUrl#url
ExploreByUrl: move redirect path building into model

This means that the controller has to do less.
Provide more appropriate status code explore invalid
Merge pull request #199 from alphagov/tidy-explore-page

Tidy anonymous feedback explore page