[MCKIN-5773] When running on mobile, make the item bank scrollable in the x direction without wrapping the items

reviewer changes

[MCKIN-5774] Wait 500 milliseconds before starting drag on touch.

It can be confusing when the outline is drawn around the item that you
just tapped, even though the item is not draggable at that point.

This allows users to scroll the item bank with their finger without
accidentally starting to drag items.

Mouse drag events still start without a delay, since scrolling works
differently on desktops.

I found no way to implement this with jQuery UI draggable, so I had to
reimplement drag and drop interactions, which in some ways makes it
cleaner because it doesn't bypass virtualDOM.

Enforcing XBlock variable types

v2.0.18

Sstudent/tnl 6753

new release 2.0.17

Aj/tnl 6668 keyboard help focus

view should always show a weighted grade

put scorable first

bug fix

implements scorablexblockmixin and moves drag and drop v2 to use raw grades.

for TNL-6593

Fix typo in README.md

FIxed `test.po` -> `text.po`

Changed the order of the submit button to match capa action bar on pl…

Fix typo in README.md

Corrected "exSisting" typo

Makes draggable items more visible in IE 11

Changes the outline-style to dotted to improve visibility.

Also, IE 11 doesn't support outline-offset, which is what we use to ensure that
the draggable item outline is visible against the background.

So we've used an inset drop-shadow for normal mode.

removed duplicate msgids

a11y Interaction Improvements

See:
 https://github.com/edx-solutions/xblock-drag-and-drop-v2/pull/112#issuecomment-272210933