Make panel scrollbar track persistent

The bar is effectively invisible, but having it present means that we
don't see a jarring change in width when the content expands into the
overflow zone and triggers the scrollbar to suddenly appear.

// FREEBIE

stylesheets/_conversation.scss

@@ -21,7 +21,7 @@
 
   .panel {
     height: calc(100% - #{$header-height});
-    overflow-y: auto;
+    overflow-y: scroll;
 
     .container {
       padding-top: 20px;

stylesheets/manifest.css

@@ -640,7 +640,7 @@ input.search {
   height: 100%; }
   .conversation .panel {
     height: calc(100% - 64px);
-    overflow-y: auto; }
+    overflow-y: scroll; }
     .conversation .panel .container {
       padding-top: 20px;
       max-width: 910px;