[FIX] website_event: clean local events snippet before save

The local events snippets is an empty HTMLElement which is filled by
JavaScript on page loading. Before this commit, the filled content was
saved in database. This does not create a real problem as the content
is reloaded as soon as the page loads but you can see old events
flickering before the new events are actually loaded.

Now the snippet is cleaned before saving. This also allows to add
default content with a custo so that it is shown during the events
loading (as we are currently doing on odoo.com...).

closes odoo/odoo#35597

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>