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 wether this could be implemented with a special syntax that would later be parsed and replaced, although hopefully there's a simpler solution.