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
Upgrade gds-sso to 9.2.5

- This fixes a problem where we were seeing duplicate users created in
  Preview when we signed in to Transition, because Transition's
  database is synced to preview each night but Signon's isn't. The SSO
  gem wasn't recognising us as already having a user, but this bug was
  fixed in this later version of the gem.
- All our tests still pass, so this looks good.
Merge pull request #308 from alphagov/update_gds-sso

Upgrade gds-sso to 9.2.5
Upgrade admin gem to 0.1.0

* Fix broken footer html
* Fix visited link styles on bootstrap components
* Get environment indicator styles
Add custom Transition favicons
Use gem's "link-inherit"

* Replaces the more specific glossary link style
Merge pull request #305 from alphagov/environment-indicator

Show environment indicator
Avoid counting things for summary

* Use of #any? was triggering a count query
* Use of .all 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