Remove GORM from codebase

[ChrisSchinnerl]

This probably won't be done with a single PR but I think we might be able to get rid of GORM in small steps if we replace one method after another with the standard sql driver.

The goal is to create stores/sql/sqlite and stores/sql/mysql sub-packages with implementations for a common low-level interface to use in stores.

Tasks

• Replace gormigrate
• Replace gorm