Add main edition type to admin edition list

Also added subtype to edition show pages because it
uses the edition_type helper.

https://www.pivotaltracker.com/story/show/57977482
Add snippets db table
Add method for adding html snippets to ministerial roles page
Merge pull request #918 from alphagov/ministerial-reshuffle-message

Ministerial reshuffle message
Merge pull request #912 from alphagov/57977482-add-main-type-to-admin-edition-index

Add main edition type to admin edition list

https://www.pivotaltracker.com/story/show/57977482