Add failing cuke for corp info page attachments

Also extract some reused code from policy advisory group helper methods.
Use ID as param for corp info pages in admin

In the frontend we want to use slugs. In admin we want to use IDs. This
removes the default #to_param from CorporateInformationPage and adds a
simple SlugToParam module that can be used to put the functionality back
in the frontend.
Merge pull request #1168 from alphagov/fix-attachment-problems

Additional fixes for https://www.pivotaltracker.com/story/show/53757477

Fix attachment editing for corporate information pages