[REF] web_editor, *: use the name of invisible snippets in the UI

* website, website_mass_mailing

Instead of a custom string, use the name of the snippet when displaying
an 'invisible' snippet in the 'invisible elements' section.

This commit also fixes a font-awesome bug introduced with the library
update revert.

Part of https://github.com/odoo/odoo/pull/41789



closes odoo/odoo#41789

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