rename public_key in public_keys

remove compatibility layer to convert public keys newline delimited
in json list

sql/mysql/20190807.sql

@@ -0,0 +1,6 @@
+BEGIN;
+--
+-- Rename field public_key on user to public_keys
+--
+ALTER TABLE `users` CHANGE `public_key` `public_keys` longtext NULL;
+COMMIT;