Filter mappings by tag when clicking a tag

* Clicking a tag shows all mappings tagged with that tag
* Clicking another tag shows all mappings tagged with both, and so on
Use ActsAsTaggableOn delim for splitting/joining
Highlight tags that are being filtered by

* Split out filter_by_tag_path into more useful parts
* When selecting a tag, on the subsequent listing it should be
highlighted
Show filtered tags in page header

* Include a list of tags that we’re filtering by, with the ability to
click and remove them, one by one
Break json part of most_used_tags out

* Make the most_used_tags part re-usable by things that don’t need a
JSON representation
Add top tags to site dashboard

* Use top tags from site rather than overall
Re-phrase mapping actions on site dashboard
Update breadcrumb when mappings filtered
Update page titles when filtered

* <title> and <h1> should also reflect the filtered state
Update dashboard and analytics titles too

* Put the most important part first
* Include analytics in the title when using the tabs
Merge pull request #181 from alphagov/tag-filtering

Tag filtering