Adding html5sortable library

html5sortable is a library for making a list of items in html drag-and-drop rearrangeable (sortable). It is very lightweight, as it uses modern browsers' built-in drag-and-drop functionality.

I got this from the github repo here: https://github.com/voidberg/html5sortable
This version is a fork of the original project, found here: https://github.com/farhadi/html5sortable

I made a pull request of the fork, because it is actively maintained and more feature-rich.

The newest source is available from https://github.com/voidberg/html5sortable/archive/master.zip.
The version used in this commit is available from: https://github.com/voidberg/html5sortable/archive/1bd25ffc5d91e43f076764c706308dab5f648c3f.zip

Evidence of popularity: The original project (https://github.com/farhadi/html5sortable) has 900+ stars and 200+ forks. This fork has only 73 stars, but is by far the better version in my opinion. It can do everything the main version can do, with fewer bugs and more options.