This updates the file attachments API update endpoints to allow undoing the

pending deletion of file attachments. We add a pending_deletion query

parameter that can only be set to False to do this.
This also reduces the number of queries performed when deleting multiple

revisions of file attachments. Instead of removing revisions one by one,

we do a batch remove which reduces this to just one query instead of

one query per revision.

Ran unit tests.
Used in an upcoming change.