Consider removing the custom filearea and reusing the core

[brendanheywood]

Core already does image resizing for the image previews. The image previews correctly handle the capability checks already.

https://github.com/moodle/moodle/blob/master/lib/filestorage/file_storage.php#L224-L235

The core sizes eg 'thumb' used by the preview won't work for this plugin, both because they are too small, but also because they are always a square. But as long as the custom size is in the filearea then core will still load it correctly for free:

https://github.com/moodle/moodle/blob/master/lib/filelib.php#L2618-L2621

This will also make is much easier to implement #8 and the remained of #7