Add FileAttachmentHistory model and evolutions.

Review Request #6385 — Created Sept. 30, 2014 and submitted — Latest diff uploaded

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Review Board
master
58dd8b1...

Reviewers

This change adds a model which is used to aggregate multiple FileAttachments
together into a single "history". This will be used by future changes for the
"Update" command, allowing users to replace attachments with new versions.

Ran evolutions.