v2.9.0

Added

• Add enrollment count to course run and to course page
• Add blocks on header and footer for branding overrides
• Add organization logo to course glimpse

Changed

• Update course search API to remove MPTT regexes as query params and replace
  them with arrays or strings.
• Use stable IDs for categories and organizations in ES indices and throughout
  all search-related code.

Fixed

• Fix useCache hook when it has to encode UTF-16 characters to base 64.
• Fix an issue related to css selector priority from r-scheme-colors mixins
• Fix css classes related with course runs on the course detail page.
• Fix error on course page when course run doesn't have any title and the
  setting WEB_ANALYTICS_ID is enabled
• Fix organization str when organization code is null