Replace the old course/dragdrop.js file (which was not even minimised)
to AMD modules and integrate them to the new reactive course editor.
From now on, a file can be drop over any course section, no matter if it
is in the course content or in the course index. It will also start
using the new process monitor to show the uploading state to the user.

In 4.0- version each time the course page is loaded the file handlers
are calculate din the backend and injected directly into JS using a json
encapsulation. With this new webservice the handlers can be obtained
directly from the frontend when needed.

This commit adds all the necessary CSS and logic to handle file dropping
into a reactive compoment. From now on, a reactive application can
handle both element drag&drop and file drop easily.

Create a new UI compoment to queue, execute and display errors on batch
processing. The first use of this component is when the teacher drops a
file into the course page.

Half of the times the normalise module is used is to get a single
element. However, because jQuery elements can contain multiple elements
the getList is always an array. Due to this in many ocasions we repeat
the getList(VAR)[0] line instead of having a more readable getFirst
method which only implies a couple of lines in the original code.

The behaviour-specific data was getting corrupted when the regrade
recreated the first step, because $oldstep->get_behaviour_data() was
stripping off the leading '-' characters from the names, and they were
not being added back.

Make sure that the $required param for external_description and its
subclasses are either VALUE_DEFAULT, VALUE_REQUIRED, or VALUE_OPTIONAL.

Some calls to the external_multiple_structure's constructor incorrectly
pass parameters where the value for the $description parameter is not
being passed. This results to invalid values being passed for the
$required parameter.