and modify course_navigation correspondingly

  - uses multicourse settings (default_chapter and default_section)
  - it would be nice to use course_settings instead, but that is not
    currently compatible with multicourse

 - issue is that content machine isn't whitelisted for email
 - this also provides secondary filtering for user registration

  - django debug commented out in lms/envs/dev.py (temporarily - need cleanup)
  - email backend set
  - index page modified for edx4edx

uses new 'github_url' dict key in multicourse_settings
this replaces the previous "quickedit" functionality