Related to #2235.
This is done by:
1. Delaying the notification sending after the SSH key is commited in DB
2. Gracefully exit the mailer method if the record cannot be found
When an email notification concerns a specific object (issue, note,
merge request, etc.), add a link to the footer of the email that opens
the item's page in a web browser.
Rationale:
* The link is predictable: always the same text, always at the same
location, like any reliable tool.
* It allows to remove the inline-title in many emails, and leave only
the actual content of the message.
Emails are used to associate commits with users. The emails
are not verified and don't have to be valid email addresses. They
are assigned on a first come, first serve basis.
Notifications are sent when an email is added.
