Disable inspection of references with unknown contents

Ed & Roan:
 Disable editing of references of which we are unable to find the
 source (e.g. <ref name="x"> without a target, or when the target is
 currently nested in something we don't yet process such as inside a
 <references> block or a template).

Timo:
 Improve UI to not be a regular focusable node where the inspector just
 won't show up but add a not-allowed cursor and explanatory tooltip.

James:
 Fix messages to refer to VisualEditor instead of "the" VisualEditor.

Change-Id: Ib2bca092ce13c9187fa8b27ad6a6404cae02aea2
Merge "Disable inspection of references with unknown contents"