Previously we inadvertently interpreted indented HTML as
code blocks. This was a regression.
We now seek to determine the indentation level of the contents
of an HTML block, and (optionally) skip that much indentation.
As a side effect, indentation may be stripped off of raw
HTML blocks, if `markdown_in_html_blocks` is used. This
is better than having things interpreted as indented code
blocks.
Closes#1841.
