Rename 'admin' to 'GDS Editor'

We currently use 'admin' to mean 'can edit any site', whereas we want to add
the ability to edit things which are beyond mappings (eg the redirection host
whitelist).
Merge pull request #294 from alphagov/rename_admin_permission_to_gds_editor

Rename 'admin' to 'GDS Editor'
Add a global_type column to the sites table

- Due to doing this, make global_http_status and global_type stay in
  sync with each other by setting both attributes in previous data, with
  some manual SQL in a migration. Following @jennyd's lead with what she
  did with mappings.
- This will eventually replace global_http_status, in the unresolved
  mappings process.
Merge pull request #295 from alphagov/change_global_http_status_to_global_type

Change global_http_status to global_type
Validate New URL against whitelist in database

Whitelist copied from Bouncer.

Conflicts:

	spec/models/mappings_batch_spec.rb
Remove redundant http_status code

We've rewritten history in the versions table to pretend that it was always
Mapping.type, not Mapping.http_status.
Extract hostname validator
Add validations to WhitelistedHost
Admin UI for viewing/adding to Whitelisted Hosts
Merge pull request #297 from alphagov/remove_redundant_code_in_versions_helper

Remove redundant http_status code
Merge pull request #273 from alphagov/validate_new_url_in_whitelist

Validate New URL against whitelist in database
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.