Add Index to action.user_id (#27403)

Another Column that needs a Index. Found at
https://codeberg.org/forgejo/discussions/issues/61#issuecomment-1258744.

Co-authored-by: Giteabot <teabot@gitea.io>

models/activities/action.go

@@ -140,7 +140,7 @@ func (at ActionType) InActions(actions ...string) bool {
 // used in template render.
 type Action struct {
 	ID          int64 `xorm:"pk autoincr"`
-	UserID      int64 // Receiver user id.
+	UserID      int64 `xorm:"INDEX"` // Receiver user id.
 	OpType      ActionType
 	ActUserID   int64            // Action user id.
 	ActUser     *user_model.User `xorm:"-"`