Added $wgMWOAuthSharedUserSource to avoid ambiguity if two extensions handle the hooks

* Also throw an error if no hook sets the required values

Extensions implementing OAuthGetUserNamesFromCentralIds, OAuthGetLocalUser
FromCentralId, or OAuthGetCentralIdFromLocalUser hooks should check that
their extension is named by this parameter, otherwise they should not
attempt to setup the user.

Change-Id: I8886ab6e14897bacc2b74befe89f635ef8035c88
Merge "Added $wgMWOAuthSharedUserSource to avoid ambiguity if two extensions handle the hooks"