Review create-schemas default behavior.

Get back in line with what the documentation says, and also fix the case for
default MySQL migrations now that we target a PostgreSQL schema with the
same name as the MySQL database name.

Open question yet: should we also register the new schema on the search_path
by default?

  ALTER DATABASE ... SET search_path TO public, newschema, ...;

Is it more of a POLA violation to alter the search_path or to not do it?

Fix #582.

src/sources/common/db-methods.lisp

@@ -213,7 +213,7 @@
 			    (disable-triggers nil)
 			    (data-only        nil)
 			    (schema-only      nil)
-                            (create-schemas   nil)
+                            (create-schemas   t)
 			    (create-tables    t)
 			    (include-drop     t)
 			    (create-indexes   t)