[FIX] mail: prevent the composer to break layout in client action

The composer can grow vertically.  It will eventually grow out of the
chat content, which breaks the layout.  This commit fixes the issue.

addons/mail/static/src/less/client_action.less

@@ -112,6 +112,9 @@
         height: 100%;
         margin-left: @mail-chat-sidebar-width;
         position: relative;
+        @media (min-width: @screen-sm-min) {
+            overflow: auto;
+        }
 
         @media (max-width: @screen-xs-max) {
             margin-left: 0;