Fix some transaction issues when retitling projects
Summary:
Fixes T5530.
- We currently fail if you rename a project so it has the same slug (e.g., "Example" -> "ExAmPlE").
- We currently fail if you rename a project so one of its secondary hashtags becomes the primary hashtag.
Instead, succeed in these cases.
Test Plan: Successfully performed the renames described above.
Reviewers: btrahan, chad
Reviewed By: chad
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D9829