[feature] Allow configure default email notification settings for different orgs

[nemesifier]

Is your feature request related to a problem? Please describe.
This is a follow up of #132 and #110, the idea is to allow users to configure the default settings more easily.

In particular we shall keep the following in mind:

Defining when and how users set their initial preferences can help streamline the notification management process and ensure users are not overwhelmed from the start.

Describe the solution you'd like

We could allow to edit these settings from the organization page, like we do with the settings for the modules like radius and controller, eg:

Using the fallback fields from OpenWISP utils, would give us the bonus that the global defaults can also be changed from settings.py.

I think we should allow configuring the following:

• Email notifications enabled/disabled
• Web notifications enabled/disabled (disabling this disables currently also email notifications)
• Email batch interval, maybe to avoid overwhelming users with notifications we can default the batching to 24 hours in order to send a daily summary
• Email display limit