[feature] Notify expiration of certs and auto-renew functionality

Depends on https://github.com/openwisp/django-x509/issues/202, the goal of this issue is to override the base functionality in django-x509 which allows to notify admins and managers using the OpenWISP Notifications module, we need to disconnect the default signal receiver and supply a signal receiver which sends a notification using the notification module.

We will need to register the new notification type (we could use generic_message but it doesn't send emails so it's not great for this).

Admin & REST API

We will also need to double check the auto-renew functionality works and it's possible to set this both in the admin and REST API.

Docs

We also need to update the docs:

• mention these features in the feature list intro
• lists the settings in the settings page
• list django signals inherited from django-x509
• add a dedicated page "X509 Cert Renewal"
  • auto-renew feature for:
    • Certs
    • CAs
  • notifications for expiring and expired/auto-renewed certs

Start by reviewing the dependency on django-x509 issue 202 and the existing certificate signal and notification integrations. Check the Admin and REST API auto-renew paths, then the feature-list intro and settings documentation. Done means custom expiration and renewal notifications, a registered notification type, working Admin and REST API configuration, and the requested X509 Cert Renewal documentation.