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
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
Increase default limit of tags to show to 50

MOJ are using more than 10 tags, and since we limit the number of
tags which are shown in the dashboard and in the filter dropdown,
they had no way of getting back to the mappings tagged with the less
frequent tags. As a short-term measure we can increase the default
limit to 50, which shouldn't be unmanageable and it's likely that very
few sites have anything like this many tags anyway. As a more robust
solution we should make it possible to type in the tag filter dropdown
to find any tag, but that is a separate story.
Set global_http_status from global_type even if it is already set

Now that the site yaml import sets global_type only, it is the source
of truth, and so if a global status is removed from the site yaml the
change of global_type to nil should also set global_http_status to nil,
rather than leaving it as the original value. If it had been a redirect,
the global_new_url would have been removed by the import so the redirect
would error with the current behaviour of Bouncer (still using the old
column).
Merge pull request #304 from alphagov/increase_most_used_tags_default_limit

Increase default limit of tags to show to 50
Merge pull request #306 from alphagov/always_set_global_status_from_type

Set global_http_status from global_type even if it is already set