jQuery UI is not actually used.

The JavaScript client depends on several libraries, which are currently
tracked in our repository. This makes keeping up with updates unnecessarily
complicated and pollutes the repository.

In the past we faced the same problem with PHP dependencies and we solved it
using Composer. For JavaScript, Bower and NPM are two most common package
managers. Initially, Bower was used for its use of flat dependency tree
but in the end NPM prevailed because it is much more common in distribution
repositories.

This patch modifies the build script to install the dependencies and moves
jQuery dependency to the new system. Other dependencies have yet to come.

As with Composer, nothing changes for normal user; developers will be forced
to update the dependencies on checkout.

In addition to jQuery, we are now also installing the following packages:

* malihu custom scrollbar plugin
* jQuery Hotkeys
* jQuery Mousewheel

The list of assets is now used in both index.php and gruntfile.
To make sharing the data easier it was moved to JSON file.

FancyBox was the last of the third party JavaScript libraries. Since
the official NPM package only offers the new major version, selfoss
was also upgraded to use it. As a bonus, long standing issue with
keyboard shortcuts was fixed.

Closes: #796