use utf8mb4 as mysql connection encoding

note that this only affects `character_set_client`, `character_set_connection`,
and `character_set_results`. it does not change `character_set_database`,
`character_set_server`, and the charsets of all existing tables, which still
need to be configured/migrated separately.

Author: TomiBelan

app/config/config_local.yml.dist

@@ -13,7 +13,7 @@ parameters:
 #        dbname:   anketa
 #        user:     anketa
 #        password: ~
-#        charset:  UTF8
+#        charset:  utf8mb4
 
     allow_db_reset: true