Don't hide errors with mappings

We added this behaviour because we thought "if an error (now) has a mapping, it
has been fixed and so is irrelevant". The problem is, it means that the graphs
don't match the tables (because they are built from DailyHitTotals, which are
not filtered).
Make hits_table partials more readable

Also encapsulate some knowledge.
Reduce duplication of "hit is now" code
Indicate errors which have been fixed
Kill two bugs with one bird

* Stop @filter blowing up in the saved mappings modal because
  _tags assumes it's only ever being used from MappingsController#index
* Don't make the tags links when we're not on MappingsController#index
Merge pull request #302 from alphagov/add-mapping-from-hits-bug

Kill two bugs with one bird
Set global_type in site yaml import

We need to import sites which are still handled by redirector, so
can't switch to using redirect/archive as global values in all site
yaml files, so should convert 301/410 to redirect/archive on import.

Before this, global_http_status was being set on import but not
global_type, so any changes to global statuses would not have been
reflected in the global_type column. This wouldn't have been a problem
for Bouncer, which isn't yet using the new column, but various things
in Transition are using the new column already so for example users
wouldn't see the message on the site dashboard about the global type/status.

The import now only sets global_type explicitly; the global_http_status
is set for now by the before_validation callback.
Replace last uses of global_http_status with global_type
Remove unused step
Merge pull request #303 from alphagov/set_site_global_type_on_import

Set site global_type on import
Move http_status out of Mapping

* Into hits_helper
* Still not exactly ideal (really, this is Bouncer's concern), but it's tiny and
  used only here
Merge pull request #299 from alphagov/dont_hide_errors_with_mappings_2

Don't hide errors with mappings
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.
Don't offer "unresolved" as a filter option

- There's a separate story in the backlog to filter by unresolved.
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.