Migrate GiveWP's Migrations into Schema

Give's [Migrations](https://github.com/impress-org/givewp/tree/develop/src/Framework/Migrations) and the non-UI code for their [MigrationLog](https://github.com/impress-org/givewp/tree/develop/src/MigrationLog) would be a nice addition to Schema.

We'll need to migrate that over and convert to WP standards.