R1:3d804bf14d5d

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

Maniphest Tasks: T5458, T5530

Differential Revision…