Rerank subthoughts when sibling collision

[raineorshine]

When moving (dragging-and-dropping) a thought into a context, that context may have a thought with the same value, which may have different subthoughts (children). When this occurs, the subthoughts from the duplicate thoughts are merged.

Current Behavior

Subthoughts are combined into the destination (dropped) thought without modifying their rank, potentially creating duplicate ranks and/or resulting in an unexpected order.

Expected Behavior

The source subthoughts (drag) should be re-ranked starting with the rank of the last destination subthought (drop) + 1.

e.g. Given the following:

- A
  - B
    - x (rank 7)
    - y (rank 8)
- M
  - B
    - z (rank 4.5)

Moving /M/B into /A should result in the following:

- A
  - B
    - x (rank 7)
    - y (rank 8)
    - z (rank 9)
- M