Make it possible to create autonumbered external links from link inspector

Inserting a new external link from the link inspector when nothing is
selected (collapsed selection) will now produce an autonumbered external
link ('link/mwNumberedExternal' node, corresponding to wikitext
"[http://example.com]"). It used to produce annotated link text
('link/mwExternal' annotation, corresponding to wikitext
"[http://example.com http://example.com]" or "http://example.com").

Depends on two commits in VisualEditor/VisualEditor:
* Icc2e368a to insert unannotated content from the inspector.
* I40cd4d5a to insert non-text content from the inspector.

Bug: 51309
Change-Id: Ie4a633ed42907551337a62d96d0abd59e208d49d