Fix visited link coloring hiding subscribed status in groups list

Fixes #720 by making the color explicitly apply in the :visited
case.  (Other specificity hacks could fix this, but this is the
cleanest.)

tildes/scss/modules/_group.scss

@@ -29,7 +29,8 @@
 }
 
 .group-list-item-not-subscribed {
-  a.link-group {
+  a.link-group,
+  a.link-group:visited {
     color: var(--warning-color);
   }
 }