[BUGFIX beta] A stack of contextualElements for render buffers, clean omitted start tags

During a renderTree call, keep track of what the contextualElement is and pass this to the buffer reset. This allows `element()` to have the correct context.

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 #5565
These are shifted off on [line 110](https://github.com/emberjs/ember.js/pull/5571/files#diff-e5ee214da5b218268498cac45d16b606R110) where it seems foolish to perform the same checks again. Instead, just push the previous contextualElement onto the stack again. Open to some cleaner ideas.