A new method, get_unaddable_by_theme_block_types(), has been added to
block_manager class to let themes define, using the unaddableblocks
theme setting, the blocks that won't be displayed in the "Add a block"
list.
For now, only the boost theme uses it, to disable some of the blocks,
like settings, navigation, course_list and section_links (they are still
required for classic).