Avoid counting things for summary

* Use of #any? was triggering a count query
* Use of .to_a will make sure that #any? happens on an array
* Avoids one COUNT per summary category for a 30% speed boost
Eager load hosts and mappings
Fix a tags performance regression

* Switching to taggings for eager loading created a table
  scan on 250K row table taggings when indexing Mapping.
  Create an index to mitigate the effects of this.
* See https://github.com/alphagov/transition/commit/514f017ac2ecee7022faa603ee5c48e892d0d425
Merge pull request #307 from alphagov/performance-of-hits

Performance of hits pt1
Only validate New URL in whitelist for redirects

Other validations of New URL are only done if the type is a redirect. This
means that if you typed in something invalid for New URL and then changed the
type to Archive, it won't reject the change you actually wanted and complain
about the value of a field which we have hidden from view with JavaScript.
Merge pull request #312 from alphagov/only_validate_in_whitelist_if_redirect

Only validate New URL in whitelist for redirects
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.