Add merge request description to merge request merged email
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### Description
Gitlab sends a very brief letter if merge request is accepted.
> Merge Request !x was merged!
It would be nice to extend mail body with merge request description.
### Proposal
Add merge request description to MR merged mail.
Current mail template [merged_merge_request_email.html.haml](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/notify/merged_merge_request_email.html.haml):
```
%p
Merge Request #{@merge_request.to_reference} was merged
```
We already have this code in [new_merge_request_email.html.haml](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/notify/new_merge_request_email.html.haml):
```
- if @merge_request.description
%div
= markdown(@merge_request.description, pipeline: :email, author: @merge_request.author)
```
This will make notification more user friendly and much more informative.
issue