Remove HtmlVersion admin controller mixin

There's a name collision with the HtmlVersion class that can break the
Rails code reloader in development.

Also, we didn't really need this module.
Rename Edition::HtmlVersion to Edition::HasHtmlVersion

There's a name collision with the HtmlVersion class that can break the
Rails code reloader in development.
Edition::DocumentSeries renamed to HasDocumentSeries

There was a name collision with the DocumentSeries class that confuses
the Rails code reloader in development mode.
Rename MainstreamCategory to HasMainstreamCategories

Avoids a name collision with the MainstreamCategory class which confuses
the Rails reloader.
Merge pull request #684 from alphagov/name-collisions

Modules named after classes confuse Rails reloader