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.
