R1:33f55a85b09f

Fix project hashtag bugs: allow simultaneously changing name and adding same name as a tag

Summary:
Fixes T8509. Changes these behaviors:

- If you create a project named "QQQ" and add "qqq" as a hashtag at the same time, it fails in an unhelpful way. (Now: succeeds.)
- If you add "qqq" as a hashtag to a project with primary hashtag "qqq", it fails in a correct but probably unnecessary way (Now: just works).

We could make one or both of these behaviors show the user an error instead, but I think it's…