Package branding-alt-education: Information

- Add default edition to os-release and dconf.
- Return indexhtml to menu and desktop.

- Update links and navigation in the "About" page (indexhtml):
  + Update telegram channel link URL.
  + Update links to English pages.
  + Remove the course schedule URL because it leads to a 404 error.
  + Update feedback link URL.
  + Improve navigation and add new links.

- Add BUILD_ID, ALT_BRANCH_ID, DOCUMENTATION_UR, SUPPORT_URL,
  VARIANT and VARIANT_ID to the os-release.
- Replace hardcoded branch name with a variable in the index.html files.