[FIX] web_editor: toolbar in website forum

Steps:

In a website forum post without any previous scrolling, select some
text. A traceback appears due the fact that there's no .o_action_manager
element in website forum.

This commit uses the document body as a fallback for the scroll
container when the .o_action_manager element is not present.

task-3506312

closes odoo/odoo#135785

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>