Human-readable section ID support

Human-readable section ID support

It adds the ability to replace the current section ID escaping
schema (.C0.DE) with a HTML5-compliant escaping schema that is
displayed as Unicode in many modern browsers.

See the linked bug for discussion of various options that were
considered before the implementation. A few remarks:
* Because Sanitizer::escapeId() is used in a bunch of places without
  escaping, I'm deprecating it without altering its behavior.
* The bug described in comments for Parser::guessLegacySectionNameFromWikiText()
  is still there in some Edge versions that display mojibake.

Bug: T152540
Change-Id: Id304010a0342efbb7ef2d56c5b8b244f2e4fb2c5