Add a "tag mappings" feature
Add acts-as-taggable-on and migrate
Minimal tagging controller function
Make the feature pass:

* Switch to tag_list in the form/tests
Test the default configuration of acts_as_taggable_on

* Not testing anything we wrote, just being explicit about
  how it behaves as configured
Tag mappings when bulk adding
Move bulk success messages into en.yml
Make bulk_adder_spec idiomatic RSpec

* apropos of cleaning up the #success_message spec
Make BulkAdder append to tags, not overwrite
Render tags as list in .mapping-tags
Add individual tag styles

* Use tags in spans, links and lists
* Turn tags purple when they’ve been visited
Position tags in top right of mapping row

Known caveats:

* Existing line-break problems lead to more white space than is
necessary, the only way to deal with this is some form of smart
truncation
* When there is a very long original path and new URL the tags wrap to
a position between the path and new URL, which isn’t ideal
* Update tag list spec (wrapping class changed name)
Include help text when adding tags
Use consistent label margins in add mapping form

* This now matches the tag label/help text
Separate suggest URL and tag fields
Temporary workaround for other tests not restoring page size

* Needs fixing properly in another commit.
Add dismissible tags

* Split tags guide into parts
* Include dismissible tags
* Include tags being used as different font-sizes
  * Make spacing of tag relative to tag font-size
Put old page size back after tests that set it
Remove page size set now we're restoring defaults

* We fixed badly-behaved tests mutating page size state
  and making downstream tests fail. That means this test
  doesn't need to concern itself with setting it.