Bump version to v2.1.5

Make code more robust to handle edge cases

Do not break when a dict does not contain an key.

EDUCATOR-1832

Prevent context menu on option long tap in iOS.

Fix: in IE11, the resize detector was blocking content

Fixes failure when loading background image on IE11

If the option contains an image, iOS would pop up a menu with image
related options. We fixed a similar problem in Android where we prevent
default action on contextmenu events, but that does not seem to work on
iOS, which requires a special CSS property.

Use fixed positioning for mobile popups.

Fixed positioning allows us to maximize the usage of screen space and
fixes some issues with popups containing a lot of content and
overflowing the DnD container.

Image.height and Image.width returns 0 for SVGs loaded in JavaScript on
IE11, this uses a klunky workaround to get the height and width

[MCKIN-6121] Prevent context menu on long draggable item tap

We were invoking grabItem too early. grabItem redraws the element
already absolutely positioned, which messes with offset measuring.
It was causing items to "jump" to the left a few pixels after grabbed.

When the draggable contains an image, android would pop up the context
menu by default after a long tap, making it impossible to start dragging
items that contain images.

Version bump to 2.1.3

When running on mobile, make the target image scroll in the x direction

Measure the available width before rendering the drag container. Set
the drag-container's max-width to the measured available width.

Bump version to 2.1.2

[MCKIN-5776] Support for dynamic generation of target zones and image.

Instead of providing a custom image, the author can now specify the
number of columns and rows and a single zone size and zones arranged
in a grid of chosen size will be automatically generated, as well as
the background image.

[MCKIN-5772] Adjust feedback popup styles

[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