Public subscriptions

**Is your feature request related to a problem? Please describe.**
I need users to be able to subscribe to pages to get alerted when things go down

**Describe the solution you'd like**
an option to enable public facing subscriptions for each notification source (for me just email) and a unsubscribe flow (replying STOP or unsubscribe link in email footer)

**Describe alternatives you've considered**
Cachet

Look for notification source configuration and email sending logic. The unsubscribe flow needs a public endpoint to handle 'STOP' replies and link clicks. Check how subscriptions are currently stored and how to add a public flag. Review existing email templates to add an unsubscribe link.

由索引模型根據 Issue 內容生成。