Fix batch editor for edits which both add and remove projects

Summary:
Aggregate multiple add/remove transactions so we don't restore removed projects for a (remove + add) batch edit.

(Possibly we should do this in the TransactionEditor as well / instead, but it's fairly easy here and this is the only possible case currently.)

Test Plan: Performed a remove + add batch edit without issues.

Reviewers: btrahan

Reviewed By: btrahan

CC: cpiro, aran, epriestley

Maniphest Tasks: T985

Differential Revision: https://secure.phabricator.com/D1967