[IMP] mail: display activity counter on cells

For a record and an activity type (cell in the activiy view), a counter
is now displayed at the bottom right, that shows the number of
activities if more than one are scheduled.

Task 1894990

addons/mail/static/src/js/views/activity/activity_renderer.js

@@ -167,7 +167,13 @@ var ActivityRenderer = AbstractRenderer.extend({
                         $date.text(moment(date).format('ll'));
                     }
                     $td.find('a').html($date);
-                });   
+                    if (activity_group.count > 1) {
+                        $td.find('a').append($('<span>', {
+                            class: 'badge badge-light badge-pill border-0 ' + activity_group.state,
+                            text: activity_group.count,
+                        }));
+                    }
+                });
             }
             return $td;
         });

addons/mail/static/src/scss/activity_view.scss

@@ -41,9 +41,9 @@
                 }
             }
         }
-        .o_activity_btn > span {
-            bottom: 5px;
-            position: relative;
+        .o_activity_btn > .badge {
+            @include o-position-absolute($bottom: 0, $right: 0);
+
             &.planned {
                 color: theme-color('success');
             }