[FIX] web_editor: enable the use of the save option only on snippets

Steps to reproduce the bug:
- Add a "Table of Content" on the website.
- Try to save a "Block" of the table.
- 14.0: you get an error message.
- 15.0: it keeps loading forever.
- 16.0: you get a white page.

Users should not have the possibility to save part of a snippet. The
goal of this commit is to ensure that the save option applies on a
snippet. If it is the case, the user has the possibility to save it. If
not, the user does not have the opportunity to save it.

opw-3056913

closes odoo/odoo#107012

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