Reduce the amount of data put in the session

We store a 'return_url' for the mappings page for each site. This enables us to
return the user to where they were before eg bulk editing.

The problem with this is that cookies are limited in size. When we exceed that
size, nginx rejects the response because the header is too large, and returns a
502.

By storing the fullpath, rather than the URL, we reduce the problem. Whilst
there are options for solving it, we need to be careful about the security
implications.
Merge pull request #142 from alphagov/mitigate_session_overflow

Reduce the amount of data put in the session
Tweak mapping factory

Allow it to generate multiple valid mappings without having to specify path.

This will make the next commit cleaner.
Add Mapping.filtered_by_url
Add filtering by New URL to Mappings index
Improve semantics of "filter by new URL" form

* Use fieldsets and legend to markup heading for the radio button group
* Add a hidden label for the path
* Introduce class helpers to reset legend styles and simulate the label
margins bootstrap uses
Avoid duplication of remove filter link

* Introduce a new left margin helper
Fix visited link colour on btn-link buttons
Merge pull request #138 from alphagov/filter_by_new_url

Filter mappings by New URL
Stub the DNS lookup in import spec

This removes a network dependency for running tests. The hits_summary
feature still needs to download the charts library, which is trickier
to mock.
Fix ttl matcher

This was previously not failing when ttl was stubbed to return 0.
Merge pull request #144 from alphagov/stub_dns_lookup

Remove network dependency for DNS import tests