Context View: Do not allow context to be dragged into self

[raineorshine]

Steps to Reproduce

Case 1

- a
  - m
-f b
  - m

1. Activate context view on a/m.
2. Drag a/m~/a after a (as a sibling).

Case 2

Steps to Reproduce

- a
  - b
    - m
- c
  - m

1. Activate context view on a/b/m.
2. Drag a/b/m~/b after a/b/m~ (as a sibling).

Current Behavior

Maximum call stack exceeded.

Expected Behavior

First, moveThought should detect a potential cycle and bail if one is detected. Let's add a reducer test for this.

Second, the drop target should be disallowed and the thought should not be allowed to drop within itself.