MDL-41551 core: clean up custom block region implementation
MDL-41551 theme_base: convert base to use core_renderer::blocks and to always render block regions
MDL-41551 theme_bootstrapbase: fix a bug with bootstrapbase side-post only display
MDL-41551 theme_afterburner: convert to use core_renderer::blocks and to always render block regions
MDL-41551 theme_anomaly: convert to use core_renderer::blocks and to always render block regions
MDL-41551 theme_arialist: convert to use core_renderer::blocks and to always render block regions
MDL-41551 theme_canvas: convert to use core_renderer::blocks and to always render block regions