Normalize frameborder=no|yes to frameborder=0|1 in iframe sanitizer

[schlessera]

Fixes #2335

The logic to normalize the frameborder attribute is based on the text from https://html.spec.whatwg.org/#frames-and-framesets:

A frameset or frame element has a border if the following algorithm returns true:

If the element has a frameborder attribute whose value is not the empty string and whose first character is either a U+0031 DIGIT ONE (1) character, a U+0079 LATIN SMALL LETTER Y character (y), or a U+0059 LATIN CAPITAL LETTER Y character (Y), then return true.

Open questions:

• The actual normalization logic is a method called sanitize_boolean_digit() in the AMP_Base_Sanitizer parent class, to mirror the behavior of sanitize_dimension() which is in there, too. Should the base class regroup such logic that might be needed across sanitizer? Or would it make sense to extract them into either separate helper objects or traits?
• The name of the method is sanitize_boolean_digit()... is there a name that is clearer here? Maybe an official HTML "attribute type" name I don't know about?

[westonruter]

@schlessera Since sanitize_boolean_digit() is only being used in the AMP_Iframe_Sanitizer, perhaps best to just make it a private method on that class for now? Figuring out the best place for these methods and whether traits should be used will be part of the larger refactoring effort in #2315.