unescape-html module does not support HTML5 encodings

[Thracky]

The module used in MatrixMessageParser for converting escaped HTML characters to standard characters, unescape-html, does not support the unescaping of the HTML5 encoding of ' characters, '. Because other bridges may use this encoding (for example, matrix-appservice-irc, it results in messages being sent to discord with the HTML entity encoded ' still present.

It might make sense to use a more widely used module for this such as unescape that supports the HTML5 encoding and I can submit a PR for that if you agree with the change.