Ref in references support

When we encounter a ref tag inside the mw-data of a references
tag, we pass it off to the converter and store it as nested data.

In toDomElements we convert any nested children and write them
back to mw-data if changed.

As refs in references are invisible we exclude them when generating
the references list in ve.ce.MWReferenceListNode.

Bug: 51741
Change-Id: I31d06616849a00449df0fc77f3b33e46207cdc7f
Merge "Ref in references support"