Add ability to re-order challenges

[SaintPeter]

From @BerkeleyTrue on November 1, 2015 21:47

Copied from original issue: FreeCodeCamp/ChallengeOMatic1003#11

[SaintPeter]

Drag and Drop interface, ideally, from the challenge picker.

Reorder mode should be toggable, so you are not always able to drag an drop, to prevent accidents. Need to be in an "Edit Order Mode" to be able to move around.

Allow multi-select to drag groups of challenges at the same time (Ctrl-Click, Shift-Click). Checkbox, maybe?

[abhisekp]

:+1: