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Enforce attachment permissions

Permissions for the attachment-related controllers is restricted based
on the users access to the edition, i.e. if the user has permission
to update the edition, then they can manipulate the attachments.
Get ducks to quack together

No need to be passing in a local variable to the partial as all the
attachable things that use it respond to allows_inline_attachments?
Don't show markdown hints unless relevant
Delete redundant features

We now test this elsewhere