Overhaul jquery.wikibase.linkitem.js

Created a new module wikibase.client.PageConnector which allows
linking two pages from JS in a very nice manner without having
to care about anything. This allowed removing all of the item
handling code from linkItem.
This keeps the label handling like it was in the previous version.

Furthermore I've applied some stylistic and code cleanness changes,
like avoiding global state.
This still keeps both the module and the user interface as
simple as possible as it's already complex enough. Adding
additional features here (like user settable labels) isn't
a good idea as that would make the modules either to complex
or messy which would render them unmaintainable.

This fixes various bugs:
Bug: 50931
Bug: 54497
Bug: 55731
Bug: 57526

Change-Id: I0c0b478d5c3135357c19c6945c625a7f5990eb44
Merge "Overhaul jquery.wikibase.linkitem.js"