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: https://secure.phabricator.com/D9829