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.)