Add a node type for numbered external links

Since I0f0a826c in Parsoid, numbered external links are now empty
<a rel="mw:ExtLink"></a> tags. This means we have to put in a node type
for them to prevent them from being considered empty annotations and
getting converted to alienMeta.

MWNumberedExternalLinkNode is protected and focusable to avoid making
the link text (which isn't editable) clickable. It isn't inspectable
yet, we need to work on that.

Bug: 53505
Change-Id: I83f69695f3974089e51a84e799f31ab6ed879e05
Localisation updates from http://translatewiki.net.

Change-Id: I8d5789a9324d2db53d6f618d7dd2d041ed73a3ac
Merge "Add a node type for numbered external links"
[browser test] cursor feature was not tagged @login

Change-Id: I587ad033c93078d8e626840de3b3eedc73b83516