Detect missing unique lowercase indexes for case insensitive unique validations

[fatkodima]

Case insensitivity in uniqueness validations was skipped in this gem, but this is a very popular problem. I remember I was trapped by this at least once. And there is even a related article from Richard Scheeman with his investigation of the performance problems on production with his findings.

It adds some ugly columns parsing, but I think we can deal with it.