Replace globals $block_lab_attributes and $block_lab_config

[kienstra]

The globals $block_lab_attributes and $block_lab_config are used in the class Loader and in helpers.php.

It'd be best to not use globals, and instead move them into an OOP structure.

At the moment, there aren't any references to those globals, when searching WordPress plugins for them on https://wpdirectory.net/

If we're going to replace them, it'd be good to do that soon, before other plugins or themes use them.

[kienstra]

Ah, it looks like this issue is a duplicate of #240