Changes to accommodate 1.27

[mwjames]

• Various language changes because trying to get early access
  (during Setup) to Language::getCode violates MW's setup policy
  and is marked as such during the not completed wgFullyInitialised state.
• Ensure that DeferredUpdates are finished before validating possible
  assertions (Support for DeferrableUpdate #1435)
• Ensures wgExtraNamespaces is set to at least the canonical form
• Ensures that a ExpResource is using the canonical form

refs #1424, #1295

[mwjames]

Is there something in particular to test?

@kghbln To answer the question:

• Property NS (its canonical form) should be set no matter what and is expected to solve Property namespace disappears (1.27) #1424 and most likely ConfirmEdit blocks SMW #1406
• Switching a user language should work now like any other content in MW
• If a predefined property (goes for core and extensions) doesn't have a localized label, the canonical (== English) label is supposed to be used
• RDF will always use the canonical form (to my surprise encountered that my SPARQL RDF/Turtle would use the localized property labels and queries stopped working after I switched the user language ... what a drag ...)

PS: This would have all been easier if one could use Language::getCode early on but someone decide that for 1.27 (it worked before) you can't do that which brought all kinds of ... Anyway (for the predefined properties at least) switching languages is as easy as pressing the ULS button and labels are expected to adjust.

Namespace is a matter of the site language (not the user language). I was unable to find a way to extend wgNamespaceAliases with NS aliases at the time the user language changes.