Drag and drop lesson reordering in the metabox on edit course screen. s…

…trangerstudios#59

 * Add jQuery UI dependency to support drag and drop
 * Make Course lessons table rows draggable by calling sortable function from
 * Update Course lesson order via AJAX on stop sortable event callback
 * Remove Order field
 * Get Order from UI when add a new lesson to the course
 * Add server side AJAX endpoint to update Course order
 * Add better JSON response to show proper notifications to users on AJAX callbacks.