Handle corp info pages in attachments controller

Also expand the tests a little bit to cover the range of
find_attachable's behaviour
Add tabs for corporate information pages
Extract attachment markdown codes partial

Replace the inline attachment fields on corporate information page form
with new partial.
Remove attachments code from corp info controller

Attachments are now handled by the attachments controller so it
shouldn't have to care about anything.

Also remove the obsolete failing tests.
Tidy up attachable routing helpers
Clean up response factory

We shouldn't be using factories to build abstract responses, just the
concrete subclasses (consultation outcome and consultation public
feedback). If an abstract response really is wanted it can be built
using Response.new.

Also responses don't make sense without a consultation, so make sure we
build one in the factory.
More thorough attachments controller poly tests

Test view rendering for all the actions for all the expected attachable
types, since there was some functionality which didn't work for new
types.
Make attachments#index work for non-editions
Split attachment UI for policy advisory groups

Add tests for policy advisory groups to attachments controller, tabs to
the policy advisory groups form, and swap out the attachments fields for
the markdown reference.
Stop accepting nested attributes for attachments

Now that all attachment editing is done via the admin attachments
controller we no longer need to accept nested attributes. This removes
the nested attributes and all tests which was checking that behaviour.
Remove unused partial
Tidy up some attachment related cukes

Remove an unused method and swap to using the pdf_attachment helper
instead of hardcoding the file.
Make attachment partial links attachable-agnostic
Remove attachments code from policy group controller
Rewrite policy advisory groups attachments cuke
Restrict HTML attachments to certain models

Only consultations and publications are allowed them.
Bulk upload only works for editions
Remove unused attachment admin controller mixin

Now that all content formats use the polymorphic attachments controller
to manage their attachments this mixin isn't needed.
Merge pull request #1160 from alphagov/split-admin-attachments-ui

https://www.pivotaltracker.com/story/show/53757477

Split admin attachments UI for all content formats