(bug 38201) Deprecated wb.Site functionality related to "global" site ID in JS

The concept of a "global" site ID is just a confusing concept by MediaWiki, not required to be
represented in our data model just like that. Instead, just having a ID (getId) seems sufficient.
The previous wb.Site.getId got changed to return the actual ID (previously getGlobalSiteId)
instead of returning the Site's related language (which can still be received by
wb.Site.getLanguageCode).
All of this is necessary since we introduced the site groups which puts us into the situation that
we can have multiple sites with the same language code which would result into multiple sites which
would return the same value when calling getId() on them.

* deprecated wb.Site.getGlobalId (use wb.Site.getId instead)
* deprecated wb.getSiteByGlobalId (use wb.getSite instead)
* In PHP, the SitesModule resource loader module now adds a "id" field instead of a 'globalSiteId'
  field to the "wbSiteDetails" mw.config value. Also, the object's field names are now the ID
  instead of the language code.
* site links can now be removed again (using the right site ID in "wb-sitelink" template now)
* updated tests

Change-Id: I5bb664f9bbbf095db8c70424d5e617e5796d3af2
Merge "(bug 38201) Deprecated wb.Site functionality related to "global" site ID in JS"