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.

![Before and after](http://f.cl.ly/items/3n0P2c2v1P0y011c0D3e/Before%20and%20After.png)