Add initial implementation of grants

internal/dis/component/auth/policy/purge.go

@@ -0,0 +1,30 @@
+package policy
+
+import (
+	"context"
+
+	"github.com/FAU-CDI/wisski-distillery/internal/models"
+)
+
+func (*Policy) Provision(ctx context.Context, instance models.Instance, domain string) error {
+	// component is purge-only
+	return nil
+}
+
+// Purge purges every policy for the given slug form the database
+func (pol *Policy) Purge(ctx context.Context, instance models.Instance, domain string) error {
+	table, err := pol.table(ctx)
+	if err != nil {
+		return err
+	}
+	return table.Delete(&models.Grant{}, &models.Grant{Slug: instance.Slug}).Error
+}
+
+// OnUserDelete is called when a user is deleted
+func (pol *Policy) OnUserDelete(ctx context.Context, user *models.User) error {
+	table, err := pol.table(ctx)
+	if err != nil {
+		return err
+	}
+	return table.Delete(&models.Grant{}, &models.Grant{User: user.User}).Error
+}