Defer building of MWReferenceSearchWidget index

It can be very slow on pages with lots of references and
isn't required until you click 'Use existing reference'.

What is required is knowing if the list will be empty for
greying out the button, but that can be precomputed very cheaply.

Change-Id: I56909801a5685bb04e0c83cfb95463f705b8dfae
Merge "Defer building of MWReferenceSearchWidget index"