Mutiple Custom ConstanceAdmins

[ChristophRob]

Describe the problem

I want to make two forms for different constance settings, but it does not work to just set fields with "exclude" or "fields" of ModelAdmin

Steps to reproduce

from constance.admin import ConstanceAdmin, Config

class DesignAdmin(ConstanceAdmin):
    class Meta:
        fields = ('favicon', 'logo', 'icon')

class ConfigAdmin(ConstanceAdmin):
    class Meta:
        exclude = ('favicon', 'logo', 'icon')

class Design(Config):
    class Meta(Config.Meta):
        proxy = True
        object_name = 'design'
        model_name = module_name = 'design'

    _meta = Meta()

site.register([Config], ConfigAdmin)
site.register([Design], DesignAdmin)

System configuration

• Django version: 2.0
• Python version: 3.x