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

[nemesifier]

Depends on openwisp/django-x509#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

[kankaa16]

Hi @nemesifier !
I can integrate OpenWISP's Notifications, add auto-renew via the admin panel and REST APIs, and update the docs accordingly.
If it’s available, I’d be happy to take ownership of it.
Thanks!