Only CSV attachments on editions can be previewed

This will prevent the "View online" link being shown for CSV
attachments that are embedded inline on PolicyAdvisoryGroups or
CorporateInformationPages
Add inverse relationship on attachable

A minor performance tweak that means attachments don't need to reload
their attachable thing if they are instantiated via the attachable.
All edition types can have CSV preview

The preview page used to assume that it was only consultations and
publications that could have CSV previews.
Remove redundant code
Explicitly enable public action request formats

Instead of throwing 500 errors for requests for formats that are not
supported, return a 404. Actions that need to respond to formats other
than html must now explicitly enable them using the
enable_request_format helper.
Merge pull request #1093 from alphagov/restrict-csv-preview-to-editions

Restrict csv preview to editions

https://www.pivotaltracker.com/story/show/60067112
Merge pull request #1090 from alphagov/61203922-handle-invalid-json-requests

Handle invalid json requests

https://www.pivotaltracker.com/story/show/61203922