[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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[DB] Ensure forgejo migration up to date (squash)

- Hook Forgejo's `EnsureUpToDate` to Gitea's `EnsureUpToDate`, such that
the Forgejo migrations are also being checked to be up to date.
- I'm not sure how I missed this and if this has caused any problems,
but due to the lack of any open issue about it it seems to not be a big
problem.

(cherry picked from commit 6c65b6dcf6)
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[DB] Add test for TestEnsureUpToDate (squash)

- Add a test for the behavior of `EnsureUpToDate`, to ensure it will
error when needed and succeed when the forgejo version is up to date.
- Add forgejo_migrations package to GO_TEST_PACKAGES, to avoid running
it with `test-unit` and instead test it with `test-*-migration`.

(cherry picked from commit b172a50691)
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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"
@@ -595,7 +596,7 @@ func EnsureUpToDate(x *xorm.Engine) error {
 		return fmt.Errorf(`Current database version %d is not equal to the expected version %d. Please run "gitea [--config /path/to/app.ini] migrate" to update the database version`, currentDB, expected)
 	}
 
-	return nil
+	return forgejo_migrations.EnsureUpToDate(x)
 }
 
 // Migrate database to current version
@@ -659,5 +660,7 @@ 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)
 }