Fixes (follows-up I1b48ef5240, I6daff5c596):
* Invalid html passed to jQuery constructor.
* Use prop() instead of attr() for boolean values.
* Use append() instead of html() when appending nodes instead
of parsing html.
* Rename shadowed variable name clash 'mw' to 'mwData'.
* Fix odd construction where we parse '{}' to create an empty
object.
* Have ve.ce.MWReferenceListNode#update perform changes off-document
in a detached tree.
* Fix deep property access that can fail. mwData is set to
either JSON parse of data-mw attr or empty object.
Accessing mwData.attrs.group needs to be guarded by whether
mw.attrs is indeed set.
* Have `mw` and `about` attribtue in references list roundtrip
(especially mw which can data we aren't editing/re-creating).
* Add missing 'refGroup' property to MWReferenceListNode's
data element (similar to what MWReferenceNode already has).
Change-Id: I67e4f378ccd04e97361d8e58ae57db5353075756
