Request to use ElementTagProcessor in Halo 2.20 to process element tag

[JohnNiang]

Using ElementTagProcessor is more efficient than resolve whole HTML. In Halo 2.20, we respond HTML with chunked, the previous way may not work very well.

See halo-dev/halo#6670 for more.

[wenjing-xin]

Thank you for the suggestion, I'll deal with the A tag like this in the future