Introduce a concept of "non-urgent" toasts

This is somewhat expected to be temporary.

res/css/_components.scss

@@ -15,6 +15,7 @@
 @import "./structures/_MainSplit.scss";
 @import "./structures/_MatrixChat.scss";
 @import "./structures/_MyGroups.scss";
+@import "./structures/_NonUrgentToastContainer.scss";
 @import "./structures/_NotificationPanel.scss";
 @import "./structures/_RightPanel.scss";
 @import "./structures/_RoomDirectory.scss";