Add start/end tag widths for mw:WikiLink and mw:ExtLink nodes.

* Till now, all a-nodes were getting assigned null widths for
  start and end tags.  This introduced whitespace RT errors on
  code like: [[Foo|  ''bar'']] which used to RT as [[Foo|''bar'']]

* This patches computes start and end-tag widths for mw:WikiLink
  and mw:ExtLink nodes -- the specifics are documented in code.
  This also required tweaking targetOff key in data-parsoid for
  mw:ExtLink.

* By adding start and end-tag widths for a-tags, separator code
  will use original source to RT the white-space in this example.

* No change in parser or selser tests (TODO: Add tests as above).

* This fix should improve RT testing results on the RT server.

Change-Id: Id4b85e5d9b9d4ed5c10fd87095d9f1695d116ee3
Merge "Add start/end tag widths for mw:WikiLink and mw:ExtLink nodes."