Upgrading: improve unserialize->json conversion

Let the Admin download a CSV file with a list of errors when converting
unserialized data to JSON during upgrade, instead of forcing them to
fix issues one-by-one and run the upgrade N+1 times (N being the number
of conversion errors).

Also includes some code cleanup / improvements

• Conversion from legacy Database API to DbQuery
• PHPStorm static analysis warnings
• unused parameter in db_unixtimestamp()

Fixes 0028918, PR https://github.com/mantisbt/mantisbt/pull/1771