HTML API: Prevent `WP_HTML_Tag_Processor` instances being unserialized and add some extra logic for validating pattern and template file paths.

Props dmsnell, xknown, jonsurrell, peterwilsoncc, johnbillion.


git-svn-id: https://develop.svn.wordpress.org/trunk@61880 602fd350-edb4-49c9-b593-d223f7449a82