MDL-41511 blocks: improved how custom block regions were being rendered.

There is a new theme property 'blockrendermethod' that can be set by the
theme in its config.php and tells Moodle what method it is using to render
blocks in the layout files.
Either blocks, or blocks_for_region.
Then when adding custom block regions to a page content we ensure we use
the same method the theme is using elsewhere.

This is really a hack becuase we (I) didn't properly deprecate
blocks_for_region when I added the blocks method.
MDL-41551 blocks: added tracking and recognition of custom block regions
MDL-41551 theme_base: improved custom block region handling for base