[16.0][OU-FIX] website: Do BS conversion with sanitization override + Add website editors to sanitization override group

[pedrobaeza]

• Certain fields had sanitize=False or other sanitization disablings (like sanitize_attributes=False or sanitize_form=False), so when converting to BS 5, we should allow to save the result without re-sanitizing such content. For that, if the user is not on the sanitize overridable group, we add it, removing it after conversion.
• Website editors that come from v15, where there was no sanitization, would expect the same on the new version, or worst: they introduced some content like iframe in 15, and re-editing that content in 16 leads to lose it when saving it. Thus, let's add all of them to the override sanitization group for keeping the same behavior.

@Tecnativa