[api] add the possibility to subscribe to events

You can subscribe globally or on project/package leel if you want to
get notifications (not yet implemented obviously) for events either for

 - none (overwriting more global defaults)
 - project maintainership
 - package maintainership
 - all
[api] start of an action mailer for events
[api] add current hermes templates in erb format
[api] move the notification templates in the right place
[api] actually send the mail to subscribers
[api] test the emails sent out
[api] remove events for review/request accepted/declined/revoked

Noone subscribes to these, statechange/change offer the same
[api] decouple the event types from hermes type
[api] send mails to reviewers and test send mails