R1:92912a60720c

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…