Cleaner headers in Notification Emails

Make the informations available in the notification email headers (sender, recipient, subject, etc.) more readable and meaningful.

* Remove the email subject prefix
* Don't write the project namespace in email subjects
* Write the issue/merge request title in the notification email subject
* Make the email appear as sent from the action author (the actual email address is still `gitlab@gitlab.com`)

For instance, this is the notification email for a new issue comment before:

> From: gitlab@gitlab.com
> To: myemailaddress@gmail.com
> Subject: GitLab | GitLab HQ / GitLab-Shell | New note for issue #1234

And after :

> From: Nick Brown <gitlab@gitlab.com>
> To: myemailaddress@gmail.com
> Subject: GitLab-Shell |  Add local update hook  (#1234)

The recipient of the notification can easily get the gist of the message without even opening it — just by looking at how it appears in her inbox. None of the actual email addresses (From, To, Reply-to) changes, just the display name.

Having a consistent subject for all notification emails sent about some resource also allow good email clients to group the discussion by thread (although grouping in Mail.app still needs some work).