Description templates for milestones
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### Description
When creating an issue there currently is an option to use description templates.
For some people when creating a milestone certain informations always have to be provided.
e.g. for a new version there might be a checklist with actions to perform before the milestone can be completed.
As a consequence it might be quite handy to have description templates for milestones, juste like we have for issues and merge requests.
### Proposal
Add description templates for milestones
### Links / references
[Docs for description templates](https://docs.gitlab.com/ee/user/project/description_templates.html#creating-issue-templates)
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