Change file attachment upload to use a backbone view.

This change adds a new backbone view for file uploads (the one that you see
when you click Update -> Add File). That popup was previously done via
$.formDlg and the old "datastore" code. Now it's a standalone view.
This will also be used for the future attachment updating feature.

Testing Done:
* Uploading new attachments.
* Error when no attachment is specified.
* Multiple errors returned from server side (see screenshots).

Reviewed at https://reviews.reviewboard.org/r/5911/