Fix project hashtag bugs: duplicate tags, uppercase tags

Summary:
Ref T8509. This fixes three issues:

• Adding a slug like UPPERCASE would not give you a normalized slug. (Now: normalizes as uppercase.)
• Adding a slug like UPPERCASE would allow you to give two different projects the different tags UPPERCASE and uppercase (and UpPeRcAsE, etc). (Now: second tag is rejected as a duplicate.)
• Adding multiple identical or similar slugs would produce a duplicate key exception. (Now: ignores the duplicates.)

Test Plan:

• Added test coverage.
• Made tests pass.
• Hit these cases in the UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8509

Differential Revision: https://secure.phabricator.com/D14870