[FIX] web: fix issue on activities with overflow on mobile

Due to o_kanban_mobile class which adds text-overflow to all span tag
overflow: hidden is applied on activities. To avoid this overflow issue
we have to use overflow: inherit property.

addons/mail/static/src/less/mail_activity.less

@@ -50,6 +50,7 @@
         .o_activity_btn {
             span.fa {
                 vertical-align: -35%;
+                overflow: visible;
             }
         }