Remove attribute-embedded HTML

[csuwildcat]

Embedding chunks of HTML in attributes is a very bad practice, we should not do this with any component. The answer is to use a few tailored methods to provide a window into Shadow DOM - it should use Shadow DOM when present, and allow content manipulation in a backwards compatible way when it isn't.

I would be sad to see this go out in this state - we should make one change: make the label write to textContent instead of innerHTML.