New process of adding and ordering material sections in the edit eventgroup

[alasdairwilson]

Currently the interface for adding sections to an event is horrible, this adds a cool new autocomplete thing to both filter and multi select sections (then press add sections) it also adds a drag and drop interface to reorder the sections.

from:

to:

This makes it far easier to add material to an event,

Finally, the ordering of the list is now done via the "files" array in the md front matters so we are not responsible for the insane order some of the courses have. I will go and fix these when I implement #204 because the idea for this is to make the files list a list of lists where each list is a "path" through the material so we have a much clearer heirarchy.

closes #212 closes #213

[martinjrobins]

this looks very cool, and will make this a lot easier! thanks @alasdairwilson