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).
* Parses the scss and fails validation if a parse error is found
* Does not detect the situation when variables are not present - this
would involve us parsing the entire tree and would be slow. It could
also change over time, depending on whats defined in the scss
from themes.
* Introduces a new admin_setting_scsscode to do this
