Dry up Attachments controllers

This is a first pass at drying up attachments controllers across the board. I have taken a look at the ManualDocumentAttachmentsController to see if it would be reasonable to dry this up too however it feels that it is different enough to the others that we can probably accept that it is different from the other AttachmentsControllers. I am open to discussing people thoughts on this. Perhaps we could do some work to allow ManualDocumentAttachments to use the same views as the other attachments do although there are some differences here such as the breadcrumbs.
We should use `private` here (and in the subclasses). Ruby's `protected` doesn't work the same as that of other languages like, eg, Java, and is mostly confusing. We have a few other uses of `protected` in this code base that we should swap to `private`.