theme: Move .url-highlighter into _message-list.scss

It's used only in messages.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/938

data/theme/gnome-shell-sass/widgets/_message-list.scss

@@ -90,6 +90,11 @@
   }
 }
 
+// URLs in messages
+.url-highlighter {
+  link-color: $link_color;
+}
+
 /* Media Controls */
 .message-media-control {
   padding: $base_padding * 2 $base_padding * 4;

data/theme/gnome-shell-sass/widgets/_misc.scss

@@ -1,6 +1,3 @@
-// URLs
-.url-highlighter { link-color: $link_color; }
-
 // Rubberband for select-area screenshots
 .select-area-rubberband {
   background-color: transparentize($selected_bg_color,0.7);