Fix init order of experimental settings for lib.

Experimental defaults are now defined in WikibaseLib.defaults.php
instead of WikibaseLib.experimental.php. WikibaseLib.experimental.php
is evaluated before the settings array is initialized, causing the
experimental defaults to be lost.

This change also solves the problem that WikibaseLib.experimental.php
needed to decide if it needed to apply the defaults to the client or
the repo sttings.

On the down side, this change means that experimental config is now
more spread out.

Change-Id: I4b3494b53e6a700d28e651b2c7f65b7ea04a309d