Support Cleaner.validate (verify, etc.)

[DylanYoung]

Enhancement:

We want to reject dirty input rather than cleaning it. It seems painful to accomplish this by subclassing cleaner.

Curious if this could be either built-in or a hook added "on_content_change" that would be called whenever the content is changed from the original value due to bleach rules (this probably isn't the best approach as it would slow down the other uses of bleach, maybe on_first_change, though I think it would be better to build this in).

Happy to look into a patch if you're amenable :)

[DylanYoung]

This was a duplicate and already rejected. I still think it's a very good idea and don't really understand the comments in the original issue.

Proposed solution:
Split the Sanitizing Filter into two:

1. Normalizing Filter
2. Sanitizing Filter

When verify mode is active, simply raise an exception from Sanitizing Filter whenever it makes a change. Catch that exception in the validate method and return False or return (False, normalized_html)

[DylanYoung]

Duplicate of #109

[DylanYoung]

Currently, we use something like the following as a partial implementation (inefficient: double parsing) of the above:

class BleachNormalizer(object):
    def __init__(self):
        self.parser = html5lib_shim.BleachHTMLParser(
            tags=None,
            strip=False,
            consume_entities=True,
            namespaceHTMLElements=False,
        )
        self.walker = html5lib_shim.getTreeWalker('etree')
        serializer_kwargs = dict(
            std_serializer_kwargs,
            # The normalizer doesn't use bleaches alphabetizer, maybe we should?
            alphabetical_attributes=True,
        )
        self.serializer = html5lib_shim.BleachHTMLSerializer(
            **serializer_kwargs
        )
        self.filters = [NormalizingBleachFilter]

    def normalize(self, html):
        dom = self.walker(
            self.parser.parseFragment(html)
        )
        for f in self.filters:
            dom = f(dom)
        return self.serializer.render(dom)

If I get more time to work on this, I'll be happy to propose a patch / forked project (as suggested in #109 ).