Deprecate *_config() functions in core.

This patch switces all known instances in core that calls any of the functions load_config(), get_config(), set_config() and del_config() into direct calls to Config::Load(), Config::Get(), Config::Set() and Config::Delete() instead.

This makes for a pretty huge patch, but as the original functions in include/config.php are mere wrappers for the static functions in the Config class, I don't think there should be any averse effects, as long as I have not mistyped anything.

We still need to keep the wrapper functions available for a time, as there's no way of knowing what other code/extensions use them. In time however, I hope we can remove the wrappers, and direct everyone to only use the Config class.

Any new code after this should use the Config class directly!