Partially completes #1068, and will allow mobile clients to drop
their separate diff-match-patch imports. Does not yet try to handle
case folding or combining-char stripping, and leaves some of the outer
HTML wrapping up to the frontend for now.
The logic for rendering the provided string has changed: missing chars
are now only inserted if they follow a correct section, and the original
text is shown instead of hyphens. This is an experiment, and can be
changed if it's not well received.
Python's regex engine performs pathologically on regexes like
'<!--.*?-->' when fed a large string of repeating '<!--' clauses.
Thanks to JaimeSlome / security@huntr.dev for the report; closes#1380.
Solved by switching to the Rust implementation, which does not suffer
from this issue.
entsToText(), minimizeHTML(), and the old regex constants have been
removed; they do not appear to be used by any add-ons.
