[DB] Forgejo database migrations

- Implements https://codeberg.org/forgejo/discussions/issues/32#issuecomment-918737
- Allows to add Forgejo-specific migrations that don't interfere with Gitea's migration logic. Please do note that we cannot liberally add migrations for Gitea tables, as they might do their own migrations in a future version on that table, and that could undo our migrations. Luckily,  we don't have a scenario where that's needed and thus not taken into account.

Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/795
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models/migrations/migrations.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"fmt"
 
+	"code.gitea.io/gitea/models/forgejo_migrations"
 	"code.gitea.io/gitea/models/migrations/v1_10"
 	"code.gitea.io/gitea/models/migrations/v1_11"
 	"code.gitea.io/gitea/models/migrations/v1_12"
@@ -647,5 +648,7 @@ Please try upgrading to a lower version first (suggested v1.6.4), then upgrade t
 			return err
 		}
 	}
-	return nil
+
+	// Execute Forgejo specific migrations.
+	return forgejo_migrations.Migrate(x)
 }