[FIX] mail: fix activity scheduling dropdown overflow

The dropdown for scheduling the next activity was overflowing inside the
container div.

After this commit, the dropdown is now attached to body element.
In other terms, the pop-in window to schedule an activity in the web kanban
activity widget is now visible (and not displayed anymore under previous line
in the widget tree view).

Task ID:2301262

closes odoo/odoo#56072

X-original-commit: 5ca624605571f79d1bca27c9a40286722c2a801b
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

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

@@ -3,7 +3,7 @@
 
 <t t-name="mail.KanbanActivity">
     <div class="o_kanban_inline_block dropdown o_mail_activity">
-        <a class="dropdown-toggle o-no-caret o_activity_btn" data-toggle="dropdown" role="button">
+        <a class="dropdown-toggle o-no-caret o_activity_btn" data-toggle="dropdown" role="button" data-boundary="window">
             <!-- span classes are generated dynamically (see _render) -->
             <span t-att-title="widget.selection[widget.activityState]" role="img" t-att-aria-label="widget.selection[widget.activity_state]"/>
        </a>