This adds the elements `formaction`, `data`, `ping`, `poster`.

We can't really add a test for the `<object data>`, since we never
allow `<object>` elements in the first place and we don't allow
settings exceptions for temporarily allowed elements.
Same for `poster` elements, since it's only used in media elements
and those are either all allowed or none.

Differential Revision: https://phabricator.services.mozilla.com/D78638