[FIX] mail: BS4, restore activity popover

Adapt SCSS which was BS3-specific and remove misuses of the
dropdown-item class.

Closes https://github.com/odoo/odoo/pull/24419

addons/mail/static/src/scss/mail_activity.scss

@@ -159,18 +159,20 @@
 
         .o_activity_link_kanban {
             visibility: hidden;
+            @include media-breakpoint-down(sm) {
+                visibility: visible;
+            }
         }
 
-        ul.o_activity {
+        div.o_activity {
             width: 290px;
             padding: 0px 0px;
 
-            & > li:last-child {
+            > :last-child {
                 background-color: $o-brand-lightsecondary;
-                color: gray('900');
                 line-height: 3em;
             }
-            li > ul.o_activity_log {
+            ul.o_activity_log {
                 max-height: 300px;
                 overflow-y: auto;
                 color: $body-color;

addons/mail/static/src/xml/web_kanban_activity.xml

@@ -20,7 +20,7 @@
     <span role="menuitem" t-if="_.isEmpty(records)" class="dropdown-item-text text-center o_no_activity">
         No activities planned.
     </span>
-    <div t-else="" class="dropdown-item" aria-haspopup="true" role="menu">
+    <div t-else="" aria-haspopup="true" role="menu">
         <ul class="nav o_activity_log" role="menu">
             <t t-foreach="_.keys(records)" t-as="key">
                 <t t-set="logs" t-value="records[key]" />