Add issue references to things

res/css/_common.scss

@@ -273,6 +273,7 @@ textarea {
 // TODO: Review mx_GeneralButton usage to see if it can use a different class
 // These classes were brought in from the old UserSettings and are included here to avoid
 // breaking the app.
+// Ref: https://github.com/vector-im/riot-web/issues/8420
 .mx_GeneralButton {
     @mixin mx_DialogButton;
     display: inline;