Streamline the content of notification emails
In notification emails, the actual content of the email is often buried under several blocks of chrome — and may even be truncated or completely missing. Ideally, the notification emails would be like *real emails*: a short message of meaningful text, sent from the author of the change that triggered the notification.
This MR includes the following changes to notification emails:
* Remove much of the chrome (e.g. the "GitLab" header)
* Emphasize the content (no more small, grayed-out content)
* Add missing informations to the emails (issue description in "new issue" email, file name in "diff comment" email)
* Add a consistent "View in GitLab" link in the footer
* The assignee is displayed only if someone is assigned
* Fix a rendering bug when viewing emails with [Zimbra](http://www.zimbra.com/)
We use these patches at [Capitaine Train](http://www.capitainetrain.com), and it has been a surprisingly big productivity boost for us.
c4f9dff480
