[FIX] web_editor, *: restore custom snippets thumbnails

*: website

Since [1], the thumbnails of the saved snippets were not right anymore,
all fallbacking to the default image. This is because snippets thumbs
now use svg files instead of png and the snippet saving feature did not
allow it. This commit fixes that and makes the feature more robust.

- Allow to use any original thumbnail, and not only for website app.
- Remove useless defaut image, a fallback for when the feature is broken
  is useless.
- Identify snippets by key instead of by main class (possible since
  the new `data-snippet` attribute on snippets).

[1]: https://github.com/odoo/odoo/commit/b7fe2bdae2f45372c0d538767a226a413b806f3f

closes odoo/odoo#55896

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

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