(bug 38201) Introduction of "allowedSites" option in SiteLinksEditTool

* "allowedSites" allows to set a set of sites the edit tool should allow for creating site links
* introduces SiteLinksEditTool.getUnusedAllowedSiteIds()
* adds some missing SiteLinksEditTool tests and does some cleanup there as well
* using the new feature, providing all SiteLinksEditTool instances with the right set of site links
  (depending on the group) in wikibase.ui.entityViewInit.js

Change-Id: I6524c4f1c97b072e08cec05c67e8b7e4b3b4b06d
Merge "(bug 38201) Introduction of "allowedSites" option in SiteLinksEditTool"