Turn supporting pages into editions.
Change SP admin routes and paths.
Convert the SP controller for edition workflow.
Remove the old supporting pages admin views
Rename policy_supporting_page translation key.
Rebuild the supporting pages admin form.
Adapt related policy fields for supporting pages.

This allows us to pre-fill a policy or policies.
Don't require organisations for supporting pages.

Removes the organisation mixin from Edition and
places it in subclasses that actually want it.
Only build organisations for editions which need them.
Stop initializing empty org arrays for editions.

This is no longer required by Rails when removing
organisations.
Rewrite the supporting pages mixin.

Currently used only by Policy.
Update preview button to support multiple targets.

Supporting pages will exist on multiple pages on
the front end depending on which policies it is
associated with.

This commit also provides us with a reusable
bootstrap button dropdown partial.
Show message when policy has no supporting pages
Extract PermissionsChecker mixin from Admin::BaseController
Make view and preview of supporting pages work

Also make the preview checking more secure by ensuring the
current user can see the previewed records.