Tasks: Drag and drop

Add ability to manually set task order and task status by dragging task rows.

Implementation

High level:

1. Backend - set / update task pos (PR 1)
2. Frontend - drag and drop UX (PR 2)

Steps:

1. Add task pos DB field
2. Display in task row (debug only)
3. Order tasks by pos
4. Add required backend functions (append to group, insert at pos)
5. Add seed data with correct pos
6. Update pos on task add / status change
7. Migration script for existing tasks (need to set pos)
8. Drag and drop (within group)
9. Drag and drop (across groups)
10. Final styling
11. Release! 🚀