Enrich the "change project tags" transaction in "transaction.search"
- This is a list of operations instead of a list of old/new PHIDs because of scalability issues for large lists (T13056).
- This is a fairly verbose list (instead of, for example, the more concise internal map we sometimes use with "+" and "-" as keys) to try to make the structure obvious and extensible in the future.
- The "add" and "remove" echo the *.edit operations.
Test Plan: Called transaction.search on an object with project tag changes, saw them in the results.
Reviewed By: amckinley
Maniphest Tasks: T13255
Differential Revision: https://secure.phabricator.com/D20209