Enable users to select unresolved as a mapping type

- Add a blurb about unresolved mappings and what the point of them is,
  when the option is selected. The copy on this probably needs a work,
  and in fact I'm not even sure it needs to be here. Thoughts?
- Mappings marked unresolved added through here get added, correctly, to
  the database with a type of unresolved.
Add a note explaining what an unresolved mapping is

- The note's copy has been discussed with @fofr.
- Add this as a partial, rendered only when the unresolved dropdown is
  selected, thanks to some JavaScript, and include it in two places.
Show unresolved as archives in the mappings table, in muted style

- Add a question mark with a hover to explain the unresolved type, but
  why it is marked as an archive and greyed out.
- Based on @fofr's prototype.
Show current status when editing unresolved mappings

- Like we do for redirects and archives, and with JavaScript off.
Default mapping type to unresolved in the MappingsFromHostPaths import
Add the ability to filter mappings by type unresolved

- Initially this was a separate story, but given the unresolved
  selection appeared in this story, I thought it sensible to make it
  work. Further parts of the filtering story will be completed in a
  separate pull request.
- Rails was pluralizing "unresolved" to "unresolveds". Tell the
  inflector that "unresolved" is uncountable, so it doesn't do this.
- Unresolved is such a long word, so edit some indentation to line if
  statements up with each other.
Add a count of unresolved mappings to the dashboard

- This is the same as the redirect and archive counts, but it doesn't
  pluralize because we've disabled "unresolved" pluralizing. For
  consistency, and because I couldn't get the `link_to` to work any
  other way, still call `pluralize` in the method to generate the count.
Make the mapping type column in the table slightly wider

- Now, the Archive (?) doesn't wrap.
Refactor two Cucumber steps into one more accurately described one

- Paired with @rgarner.
Add a test for Mapping#unresolved?
Add a specific message for changing a mapping type to unresolved
Move the "currently redirects to x" span into a partial

- This reduces duplication between the new_multiple_confirmation HTML
  and the _edit_multiple_table HTML partial.
Use an existing "filter by tag" step rather than a new one
 Update mappings through the Whitehall import if they're unresolved

 - As well as archives.
Filtering: unresolved is incompatible with new_url
Add a glossary entry for unresolved

- This probably needs copyediting.
Show the percentage of unresolved mappings on the dashboard

(Paired with @rgarner on the tests - floating point stuff isn't my
forte. Use an existing prettifying method to round up the percentage so
it isn't stupidly long.)
Deal with unresolved mappings around hits

- Show a message for each hit that has an unresolved mapping associated
  with it, in the hits table.
Finalize wording of the unresolved mappings glossary entry
Make the unresolved hit text appear on universal analytics

- Also on the 404 and 301 top ten hits summary tables.