Fix: special string.replace sequences in Markdown

[eyelidlessness]

Closes enketo/enketo#777. Also closes #110, as this incorporates the change I proposed there and adds a test for the fix.

[lognaturel]

I suspected this had nothing to do with itext translations so I verified the same issue is reproducible with the text being a direct child of label:

<label>*$' is $` this $&amp; the $$ real $0 $&lt;life&gt;?*</label>

Indeed, it's reproducible. I modified the test to verify that's also getting fixed correctly and it is.

There is a loss of functionality here: someone might deliberately be including HTML entities in markdown. However, HTML entities are not supported outside of markdown so I think it's fair to call this accidental functionality.

(Side note: I do think we should consider supporting sanitized HTML at some point to match Collect.)