[bug] Runtime modification of OrganizationAdmin

[okraits]

In config/admin.py the class OrganizationAdmin is modified at runtime:

https://github.com/openwisp/openwisp-controller/blob/master/openwisp_controller/config/admin.py#L122

Those modifications don't get picked up if someone uses an overriden admin site and registers OrganizationAdmin with this overriden admin site.

This issue is similar to https://github.com/openwisp/openwisp-utils/issues/42.

[nemesifier]

~~I'm not sure if moving those lines to the base admin file would work. If it does work, that could be a solution.~~

There's no base admin file in openwisp-controller.