Fix HTML blocks nested in indented blocks

[inchang-ing]

Raw HTML blocks which start on a line indented by more than three spaces were not extracted by the HTML block preprocessor, as it could not distinguish such a start tag nested within a containing block (a list item, admonition, etc.) from an indented code block. The block parser then treated the HTML as inline content and split it into separate paragraphs at any blank lines within the raw block.

The preprocessor now extracts such indented raw blocks, removes the indentation of the start tag from the stashed content, and preserves that indentation on the line which holds the placeholder. The block parser can then make the decision based on the resulting block structure: within a containing block the placeholder is parsed as nested content, whereas an indented code block keeps the placeholder in its content. In the latter case, the RawHtmlPostprocessor escapes the stashed HTML, so indented code blocks are rendered exactly as before.

Fixes #1096

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[waylan]

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[facelessuser]

I would like to see this tested with md_in_html as well. The testing seems generally sparse for such a potentially big change. I'd like to spend some time more thoroughly vetting this and seeing how it will impact 3rd party plugins as well.