Add a "View in GitLab" link in notification emails

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.
Remove custom link to target in notification emails

Every email has a different way of showing a link to the discussion on
the website. We don't need this anymore, as we now have a standard
"View in GitLab" link in the footer of every email.
Linkify the reference to the project in the notification emails footer

Rationale:

* Consistency: the link is always at the same place.

* Streamlining: it allows to remove the header banner, now that the
project url can be accessed from another place.
Add the description to the "new issue" and "new merge request" emails

Previously the content of the issue or merge request was missing from
the email.
Remove the author name from issue notes emails

Rationale: the author name is now displayed in the email "From" field;
this information is no longer needed.
Remove the quote-style around message in notification emails

Rationale:

* In this notification email, the message is the most important content.
It should not be dimmed or smaller than the rest of the text.

* Now that all UI elements are removed, the message *is* the email. It
should be written as a standard email message, without styling.
Remove the header banner in notification emails

Rationale:

* The link to the project is already displayed in the message footer.

* Streamlining: the content of the message is better emphasized.
Simplify the default layout of notification emails

* Less margin: the content appears as the text of the message.

* Streamlined footer: everything on one line, with a small separator.

* Zimbra (www.zimbra.com) chokes on HTML tables, and doesn't display
the content of the message. Switching to a simpler layout fixes the
issue.
Make the merge request notification emails more readable

Emphasize the meaningful informations, and remove the less needed ones:

* Use "!" to identify a merge request
* Write the name of the commented file for diff notes
* Don't show the assignee in the "merged merge request" email
 Merge branch 'simplify-emails-content' into 'master'

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)