Make data-waterfall-widgetid the only source of widget id

[lawnsea]

Currently, there are two ids: the HTML id of the widget instance in the page,
and the id of the widget model on the platform. This is confusing and also
probably not what we want.

Instead, make data-waterfall-widgetid mandatory and use it for everything.
It's probably not necessary (or desirable, really) to set an HTML id, so we
don't need to figure out how to guarantee that widget ids are unique.