Add support for AMP

[westonruter]

It would be nice if the block would output an amp-mathml element when is_amp_endpoint. This could be done via a "conditionally-dynamic" block, for example at the template_redirect action it could check if is_amp_endpoint() and if so, then register a mathml/mathmlblock block in PHP with a render_callback that takes the static markup saved in the editor and convert it to amp-mathml. (Idea comes from conversation with @youknowriad.)

In the same way, potentially_add_front_end_mathjax_script() could short-circuit when is_amp_endpoint().

[adamsilverstein]

👍 thanks for the suggestion