Add rules to support MCGA legacy domain

The [legacy MCGA domain](www.mcga.gov.uk) had content in several path structures
* www.mcga.gov.uk/c4mca/* was replaced by www.dft.gov.uk/mca/*
* www.mcga.gov.uk/mca/* was replaced by www.dft.gov.uk/mca/*
* www.mcga.gov.uk/* was replaced by www.dft.gov.uk/mca/*

It is unclear what happened when these paths conflicted, however these rules are necessary to prevent MCGA needing to manage two sets of mappings on both [their own](http://www.mcga.gov.uk) and [DfT's site](http://www.dft.gov.uk).
This last `describe` block fails because it returns a 404 instead. There are three options for getting the spec to pass:
1. add a mapping for it to `site`
2. change the spec to expect a 404
3. set `global_redirect_append_path`, `global_http_status` and `global_new_url` on the site.

My preference is to add a mapping, as it's a little more specific than a 404. For example:
```ruby
      describe 'other paths use mappings' do
        before do
          path = '/hydrography'
          site.mappings.create(path: path, path_hash: Digest::SHA1.hexdigest(path), type: 'archive')
          get "http://www.mcga.gov.uk#{path}"
        end

        it_behaves_like 'a 410'
      end
```