Remove primary key EbookSourceId

There is a one to many relationship between `Ebook` and `EbookSource`, and we
don't query by `EbookSourceId`. We add a `SortOrder` column to keep the sources
in the same order as specified in `content.opf`.

lib/Exceptions/EbookSourceSortOrderRequiredException.php

@@ -0,0 +1,7 @@
+<?
+namespace Exceptions;
+
+class EbookSourceSortOrderRequiredException extends AppException{
+	/** @var string $message */
+	protected $message = 'EbookSource SortOrder required.';
+}