[FIX] mail: client action layout: scrollbar in the sidebar

There was a weird bug on chrome when there was a scrollbar in the sidebar, as
the right part of the client action overlapped that scrollbar. The problem was
seemingly linked to the use of flex, which is removed by this rev.

Courtesy of QSM

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

 // ------------------------------------------------------------------
 // Layout
 // ------------------------------------------------------------------
+@mail-chat-sidebar-width: 250px;
 .o_mail_chat {
     .o-position-absolute(0, 0, 0, 0);
     height: 100%;
 
     @media (max-width: @screen-xs-max) {
         height: auto;
-        flex-direction: column;
-
     }
 
-    .o-flex-display();
-    .o-flex-flow(row, nowrap);
-
     .o_mail_chat_sidebar {
-        .o-flex(0, 0, 250px);
+        width: @mail-chat-sidebar-width;
+        height: 100%;
+        float: left;
         overflow: auto;
         padding: 8px 0;
         font-weight: 300;
 
         @media (max-width: @screen-xs-max) {
-            flex: none;
+            width: 100%;
+            float: none;
             display: none;
         }
 
@@ -70,12 +69,16 @@
     }
 
     .o_mail_chat_content {
-        .o-flex(1, 1, 100%);
         .o-flex-display();
         .o-flex-flow(column, nowrap);
         height: 100%;
+        margin-left: @mail-chat-sidebar-width;
         position: relative;
 
+        @media (max-width: @screen-xs-max) {
+            margin-left: 0;
+        }
+
         .o_mail_thread {
             .o-flex(1, 0, 0);
             overflow: auto;