HTMLMinifiyMiddleware: Deactivate convert_charrefs of htmlmin.minify

Since the recent update of swh-web URI scheme, numerous parameters are now
provided as query ones.

Some parameters are named "timestamp" and the HTML minifier will by default
decode encoded characters, like for instance "&times" ...

As a consequence, some URLs embedded in pages were broken and thus navigation
into the archive when a timestamp is provided as query parameter.

Also escape some "&" characters to display required after that change.