Aliased mysql to mysql2 in schema client

refs https://github.com/TryGhost/Toolbox/issues/174

- this just uses the same SQL queries for certain queries when using the
  `mysql2` library as when using the `mysql` one
- we can remove the `mysql` line when we fully switch to `mysql2`

core/server/data/schema/clients/index.js

@@ -3,5 +3,6 @@ const mysql = require('./mysql');
 
 module.exports = {
     sqlite3: sqlite3,
-    mysql: mysql
+    mysql: mysql, //TODO: remove this once we switch to `mysql2`
+    mysql2: mysql
 };