[ShadowDOM] add invalid_classes setting

[mdgbayly]

Add an invalid_classes setting similar to invalid_styles etc.

This was added to make it easy to remove the scope classes added by ShadyCSS when using tinymce in an encapsulated shadow root.

[metricjs]

Thanks @mdgbayly for this PR! We have discussed it with the team and would like to accept it. However, due to the age it is in an unmergeable state. Would you like to rebase it on the latest develop branch, then we can review it properly? You will also need to agree to our CLA for us to be able to merge it. We have re-sent the CLA email - please follow the instructions within it to sign the CLA. If you can't find it, please check your spam folder.

[mdgbayly]

Closing because I don't have time to rebase this, and it may no longer be required from our perspective if we are able to switch to a version of tinymce that has better support for shadow DOM etc. I understand this is still an ongoing area of discussion particularly for inline editors. But without that support etc, by itself this PR doesn't give us much. If it's deemed a useful feature regardless by the tinymce team, more than happy for you guys to just run with it yourself. Thanks!

[metricjs]

Thanks for the update! This feature does make sense, even separate to Shadow DOM support, so I've made a note internally to revisit the idea in the future. And if you do find it is required later, please feel free to make a new PR.