[FIX] website: fix map snippet saved

This commit fixes the issue of having duplicate iframes when dropping a
custom "s_map" snippet that has been saved by the user.

This issue was introduced by this commit [1].

Steps to reproduce the bug:
- Drag and drop a "s_map" snippet onto the page.
- Save this "s_map" snippet by clicking on the custom snippet "save"
button.
- Drag and drop the saved snippet onto the page.
- Observe that a duplicate iframe is created in the snippet.

This commit adds a check to verify if an iframe already exists before
adding it when dropping a "s_map" snippet.

[1]: https://github.com/odoo/odoo/commit/88194dd94182ad1bbdcb9aa8f855f30f6369d148



task-3340644

closes odoo/odoo#122645

Signed-off-by: Romain Derie (rde) <rde@odoo.com>