Adding support for rendering blocks later, so they can be placed individually in a region template

Every region template uses {{ content }} to render the blocks in that region. But unlike node templates, where you can do things like {{ content|without('field_example') }}, the content of a region cannot be manipulated. This is because the content variable of a region is not an array of renderable elements, but just a string of rendered HTML.

We are now changing the way that the html reaches the templates by now adding ['#theme' => 'region']. This allows blocks to be individually rendered in the region.