Refactor Edition#in_organisation to use org ID

Using the slug just meant an unnecessary join to the organisations
table.
Include supporting pages in Edition#in_organisation

Supporting pages don't have direct associations to organisations.
Instead they inherit their organisations from related policies. This
commit changes the behaviour of #in_organisation to check the policy
organisations in the query.
Merge pull request #1109 from alphagov/include-supporting-pages-in-admin-when-filtering-by-org

Include supporting pages in admin when filtering by org

[Fixes https://www.pivotaltracker.com/story/show/61539712]