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

查找通知源配置和电子邮件发送逻辑。退订流程需要一个公共端点来处理 'STOP' 回复和链接点击。检查订阅当前是如何存储的以及如何添加公共标志。查看现有的电子邮件模板以添加退订链接。

由索引模型根据 Issue 内容生成。