Allow issues/merge requests templates to customize special messages
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Currently if you have a template like the following:
```
## What are the relevant issue numbers?
## Screenshots (if relevant)
```
and you create a MR for a branch matching an issue, you'll get something like:
```
## What are the relevant issue numbers?
## Screenshots (if relevant)
Closes #1234
```
since `MergeRequests::BuildService` just appends that message at the end of the description. Ideally you could specify the position of that snippet within the template.
[We discussed](https://gitlab.slack.com/archives/development/p1472245312004351) wether this could be implemented with a special syntax that would later be parsed and replaced, although hopefully there's a simpler solution.
issue