Upgrade GORM to github.com/go-gorm/gorm

[Frederik Ring]

There has been a major update of GORM which is now at v2: https://github.com/go-gorm/gorm

It comes with some nice features Offen could be using (e.g. resizing columns on AutoMigrate) and the official docs now also refer to that version, so we should definitely update at some point.

[Frederik Ring]

Closed via #485