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 likely that this behavior is just what they always want.

Test Plan:

• Added failing tests and made them pass.
• Executed both scenarios described above from the web UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8509

Differential Revision: https://secure.phabricator.com/D14871