Closes #12318: Ignore case when determining uniquess of name & slug fields

[jeremystretch]

Closes: #12318

• Introduce two new database collations for und-u-ks-level2 (case-insensitive) and und-u-kn-true-ks-level2 (case-insensitive natural sort)
• Alter all model CharFields which are referenced for uniqueness to use a case-insensitive collation in the PostgreSQL database
• Remove all varchar_pattern_ops indexes attached to these fields (not supported with non-deterministic collations)
• Alter the UniqueConstraints on the Device and VirtualMachine models to reference the name field without calling Lower()