Allow issues/merge requests templates to customize special messages
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=15768) </details> <!--IssueSummary end--> 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.
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