Course & Personal Collections: limit nesting depth

[mark-dce]

Assume that trees depth is counted as follows

- Level 0
    - Level 1
        - Level 2
        - Level 2
    - Level 1
        - Level 2
    - Level 1
- Level 0
    - Level 1
    - Level 1
        - Level 2
        - Level 2

• [x] Moving a node moves it's children (see nestable behavior http://dbushell.github.io/Nestable/)
• [x] Collections nesting should be disabled in the UI beyond 3 levels
  • [x] Trees with level 2 deep items cannot be nested
  • [x] Trees with at most level 1 deep items can only be nested under level 0 items
  • [x] Trees with only level 0 deep items can be nested under level 0 or level 1 items