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
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.
Show current status when editing unresolved mappings

- Like we do for redirects and archives, and with JavaScript off.