Migrations

Beginning in 2021 there are some rules for migrations. This is necessary because ProvHA slave machines are not able to write to the databases but have to execute e.g. filesystem related stuff.

1. every migration has to be derived from BaseMigration
2. every migration must execute either only database related actions or none (mixing of database and e.g. filesystem actions is not longer allowed; split into two migrations)
3. every migration has to set the $migration_scope class variable; allowed values are database and system