Fix horizontal scrolling in convo/message lists (#1448)

This removes some errant horizontal scrollability from the conversation list and
the message list.

// FREEBIE

stylesheets/_conversation.scss

@@ -111,7 +111,7 @@
         width: 100%;
         margin: 0;
         padding: 10px 0 0 0;
-        overflow-y: overlay;
+        overflow-y: auto;
       }
     }
   }

stylesheets/_index.scss

@@ -24,7 +24,7 @@
   .content {
     background-color: $grey_l;
     flex-grow: 1;
-    overflow-y: overlay;
+    overflow-y: auto;
   }
 }
 .network-status-container {

stylesheets/manifest.css

@@ -868,7 +868,7 @@ img.emoji.jumbo {
   .gutter .content {
     background-color: #f3f3f3;
     flex-grow: 1;
-    overflow-y: overlay; }
+    overflow-y: auto; }
 
 .network-status-container .network-status {
   background: url("/images/error_red.svg") no-repeat left 10px center;
@@ -1158,7 +1158,7 @@ input.search {
         width: 100%;
         margin: 0;
         padding: 10px 0 0 0;
-        overflow-y: overlay; }
+        overflow-y: auto; }
 
 .discussion-container {
   background-color: #eee; }