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