Basic drag-and-drop support

This adds HTML5 drag-and-drop support to the SequenceServer interface, per #85. It's not quite ready to merge yet, but I think it's at a good point for review and feedback.

Right now it only supports dragging one file at a time; we will probably want to refactor this to add support for dragging multiple files, and possibly appending files to those already dragged into the UI.

I extracted `clearNotifications()`and `showNotification(ident)` functions, but didn't yet refactor the existing notification code to use these. None of the other code in here is defining named utility functions, so I wanted to see if I should implement this in a different way first.

It also enforces FASTA format (simplistically) and a fairly arbitrary 10 MB limit on files; we might want to re-evaluate this.