Update postfixadmin migration code

The code to migrate from postfixadmin is not working at all in the current version of modoboa. This patch makes the migration script works, it was tested against a 2.3 version of postfixadmin. There are other tricks needed to make it work besides this patch but I'm not sure where would be the right place to fix those. For example all references to the `SQLHandler` logger need to be commented on the `settings.py` file for the script to run but no idea why. Or how to actually fix it. Let me know know what you think of this. Pay special attention to how the permissions are handled, not sure if it's correct. Advices are always welcome.
In the postfixadmin database I'm migrating (v2.3), domain aliases also have a row in the domain table, not sure why. So this part makes sure that if the domain exists in the alias table, it's not treated like a normal domain. Domain aliases do get migrated later in the code. I'm guessing this case does not apply to later versions of postfixadmin, but it does not hurt to have this precaution, this way we compatibility as back as 2.3.