R1:887e344c19af

Fix project page 404 after rename and removing new name from alias slugs

Summary:
When renaming a project to a slug already listed under Additional hashtags and explicitly also removing that to-become slug, accessing the project via the URL `/tag/projectname/` returned a 404 until someone added the current project name explicitly under "Additional hashtags" again.

In that case, do not remove the alternative hashtag to avoid the 404.

Closes T15636

Test Plan: See steps in https://we.phorge.it/T15636…