Configure multiple campaign backends

[mgax]

Use a dictionary à la Django's database / cache settings (and Wagtail's search backends) for this e.g.

WAGTAIL_NEWSLETTER_BACKENDS = {
    "default": {
        "BACKEND": "wagtail_newsletter.campaign_backends.mailchimp.MailchimpCampaignBackend",
        "API_KEY": "...",
    },
    "otherservice": {
        "BACKEND": "someotherservice.backends.SomeOtherServiceCampaignBackend",
        "APP_ID": "...",
        "SECRET_KEY": "...",
        # or maybe wrap the credentials inside an `OPTIONS` key
    }
}

Different models can use different backends (or maybe you can select which service to use in the newsletter panel). It would also allow passing other options which may be backend-specific without having to define a top-level setting for each option.

#50 (comment)