[BUGFIX beta] contextualElements, clean omitted start tags

During a renderTree call, look to the morphs for information about the contextualElement. Pass this to the Ember renderer, which in turn passes it to the buffer for use when generating an element.

Here I've changed the morph creation points to explicitly pass a contextualElement. This avoids the possibility of createMorph's fallback to document.body happening.

Additionally, strip omitted start tags even if the child node causing their generation is after a script tag. See the long comment in `render_buffer`.

Fixes #5476