refactor(jqLite): make HTML-parsing constructor more robust

Previously, the jqLite constructor was limited and would be unable to circumvent many of the HTML5
spec's "allowed content" policies for various nodes. This led to complicated and gross hacks around
this in the HTML compiler.

This change refactors these hacks by simplifying them, and placing them in jqLite rather than in
$compile, in order to better support these things, and simplify code.

While the new jqLite constructor is still not even close to as robust as jQuery, it should be more
than suitable enough for the needs of the framework, while adding minimal code.

Closes #6941
Closes #6926