(bug 47610) Keep default settings out of globals.

This change allows for using $wgWBClientSettings resp
$wgWBRepoSettings for all settings, while staying compatible with
sites using $wgWBSettings for everything.

Special care has been taken to make sure default values never
override explicit configuration, and repo/client settings override
"generic" settings.

CAVEAT: This breaks B/C in cases where a values is *added* to
a setting that is an array. Doing something like
$wgWBClientSettings['foo']['x'] will no longer add 'x' to the 'foo'
setting, but *set* the 'foo' setting to an array containing just the
'x' field.

Change-Id: I14422cd85a3cf0200ea89953251b78b6b8d813cb