Add support for Join as a notification service target

[nickdollimount]

Description

Join is a free push notification service that has a simple to use API.

Desired Behavior

In the Notification Settings, I would like to see the option to use Join as a notification service for push notifications from Jellyseerr.

Additional Context

Join's website: https://joinjoaomgcd.appspot.com/

When logged in, there's a button 'Create a Join API link for your device' that will allow you create a sample API URL that can be used by Jellyseerr to send notifications to.

I've used Join for several personal projects; it's simple to use and notifications arrive immediately.

Code of Conduct

• [X] I agree to follow Jellyseerr's Code of Conduct

[pythongod]

Please open a pull request.