Switch snapshots to binary data in DB.

MySQL still doesn't like UTF8 data in
MEDIUMTEXT collated as utf8mb4, so just
use MEDIUMBLOB instead.

toread.sql

@@ -35,7 +35,7 @@ CREATE TABLE `links` (
   `title` varchar(191) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
   `deleted` datetime DEFAULT NULL,
   `keywords` text COLLATE utf8mb4_unicode_ci,
-  `snapshot` mediumtext COLLATE utf8mb4_unicode_ci,
+  `snapshot` mediumblob,
   PRIMARY KEY (`id`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
 /*!40101 SET character_set_client = @saved_cs_client */;