left_sidebar: Highlight inactive streams when narrowed to.

Having active streams greyed out can be confusing to the user. This
is especially useful when the app is still fetching messages on
a reload and the active stream has no messages and is thus marked
as `inactive`.

web/styles/left_sidebar.css

@@ -146,7 +146,7 @@ li.show-more-topics {
         }
     }
 
-    .inactive_stream {
+    .inactive_stream:not(.active-filter) {
         opacity: 0.5;
     }