[MCKIN-10516][BB-3915] Reduce drag delay on mobile
We received feedback that moving items is not intuitive on touch devices. We opened 3 PoC PRs for improving it. The #208 and #209 are rejected approaches.
This increases the sensitivity of dragging items (reduced the dragging delay). This way it will be less unusual for user to wait after holding his finger on the item:
1. Install this XBlock.
2. Create `Problem -> Advanced -> Drag and Drop` unit.
3. Test this on a mobile device. I've tested this with Firefox Nightly 210331.