Rename Rails Object to ManualsFrontend

As part of the renaming of this App, this commit removes `Hmrc` from the
name.

Part of https://trello.com/c/c6kTjaZ9/99-manuals-view-a-manual-5
Merge pull request #34 from alphagov/rename-rails-object-to-manuals-frontend

Rename Rails Object to ManualsFrontend