Issue: Remove Attachment-History

While attachment edit history is still great to have, attachment history is no longer needed even though it was developed for VaultWiki 4. Since attachments are handled completely via the editor, and the links are stored in a page's source, the attachments on a particular page are already saved in the edit history for that page. Having a separate attachhistids field and attachmenthistory table are redundant.

The drawback of this is that we also need to rethink how existing attachment assignments from VaultWiki 3 are imported. While it was always deprecated, some attachments were added to articles in the standard vB way, by using the Manage Attachments button and not choosing Insert Inline. These attachments don't already exist in the pagetext, so the importer would have to create a newer revision that appends them. Likewise, existing uses of [ATTACH] need to be handled somehow, through some conversion process.