DON'T MERGE: Prototype decoupling of swirl from content

### Prototype decoupling of swirl from content
1. Courses and  course authoring tools have been removed.
2. If no courses are installed, an installation menu is presented.
 * Suggested installations are read from Courses/suggested_courses.yaml
 * Suggested courses are installed automatically by swirl
 * User may choose to be taken to swirldev/swirl_courses for other options.
3. Miscellany
 * dev mode has been removed
 * Internet unavailability is trapped and gracefully handled.
 * The local suggested_courses.yaml could be replaced by a copy at swirl/swirl_courses.