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
WIP feature to check tags append on bulk add

* they don't, currently, they get overwritten
* still using a DB check to test the taggings, needs display
Make BulkAdder append to tags, not overwrite
Render tags as list in .mapping-tags
Janky CSS for .mapping-tags
Add individual tag styles

* Use tags in spans, links and lists
* Turn tags purple when they’ve been visited