Changes fixing known issues

I made some changes that:
- allow reordering subtasks without reloading the page
- add clear visual DnD handle for dragging
- fix issue with moving subtask to the last position
- simplifies calculations of an item position before and after DnD action
- works both on Jira 5.x (with page reloding) and Jira 6.x (without page reloading)

For me now it works like a charm.

Please review. This is my first activity on GitHub so if I'm making something wrong please don't hesitate to tell me that :)