Ignore html comment when parsing blockquotes

What does this MR do and why?

Related to #390290 (closed)

When an HTML comment precedes a multiline blockquote, the parsing gets messed up. For example,

<!-- html comment -->

>>>
something
>>>

<h1>
test
</h1>

renders as

How to set up and validate locally

Copy/paste the above markdown into a comment box, and preview/save to check that it renders correctly.

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