closes https://github.com/TryGhost/Team/issues/2429
- sends email notifications to staff users when their site receives a Webmention.
- currently behind a flag, that can be toggled in the labs settings.
refs/closes TryGhost/Team#2014
- Updated preview text.
- By default email client pulls text from email body for preview. Was added spaces to prevent such behaviour.
refs TryGhost/Team#1826
- adds new service package that manages all the email alert notifications for free members and paid subscriptions
- includes email templates for free member signup and paid subscription start/cancel
- initializes staff service before members to allow managing email alert notifications
- passes staff service to members api for triggering alerts
