improvements to <link rel="stylesheet"> + mutations

[eoghanmurray]

Note: This PR (by necessity) builds on top of #1437

existing code special cased rel="preload" so that a subsequent attribute mutation from rel="preload" -> rel="stylesheet" would get picked up.

New code deals with any attribute mutation which results in us 'now having a stylesheet link'

as well as completing the TODO for when a stylesheet link href gets updatd