Fix entities when checking databse

Notes can contain invalid HTML entities, mainly because the user imported unencoded text as HTML. This doesn't pose a problem for Anki's webviews but the Rust part doesn't handle it that well, unfortunately. It would be nice if checking the database:
1. converted entities to Unicode where that's save (as the Rust crate doesn't recognise the more exotic entities),
2. encoded &s that don't belong to any valid entity.

In fact, this is exactly what happens when a field is opened in the HTML editor. Since there doesn't seem to be a Rust crate that supports advanced decoding, maybe the webtools can somehow be utilised here?