[IMP] mail: adapt responsive rules to allow swipe

Removing these margins makes swipe on 'o_mail_preview'
nicer in enterprise without having blank spaces.

Note that these rules should be in enterprise.

Task ID : 1895453

addons/mail/static/src/scss/discuss.scss

@@ -289,13 +289,11 @@
         .o_mail_discuss_content {
             flex: 1 1 auto;
             min-height: 0;
-            margin-left: 10px;
-            margin-right: 10px;
+            margin-left: 0;
             background-color: transparent;
         }
 
         .o_mail_discuss_tab_pane {
-            overflow-x: scroll;
             .o_mail_add_thread {
                 display: none;
                 position: relative;