Added close/open button to details page of milestone (#23877)

Depending on the current state, a close or open button appears.

Close https://github.com/go-gitea/gitea/issues/19083

---
![image](https://user-images.githubusercontent.com/11333925/229367898-844ff790-c725-440a-9cf7-242c7745d643.png)
![image](https://user-images.githubusercontent.com/11333925/229367873-6a77fceb-1627-41d4-9d76-1f10ba2b278b.png)

templates/repo/issue/milestone_issues.tmpl

@@ -9,6 +9,13 @@
 			{{if not .Repository.IsArchived}}
 				<div class="column right aligned">
 					{{if or .CanWriteIssues .CanWritePulls}}
+						{{if .Milestone.IsClosed}}
+							<a class="ui green basic button link-action" href data-url="{{$.RepoLink}}/milestones/{{.MilestoneID}}/open">{{$.locale.Tr "repo.milestones.open"}}
+							</a>
+						{{else}}
+							<a class="ui red basic button link-action" href data-url="{{$.RepoLink}}/milestones/{{.MilestoneID}}/close">{{$.locale.Tr "repo.milestones.close"}}
+							</a>
+						{{end}}
 						<a class="ui button" href="{{.RepoLink}}/milestones/{{.MilestoneID}}/edit">{{.locale.Tr "repo.milestones.edit"}}</a>
 					{{end}}
 					<a class="ui primary button" href="{{.RepoLink}}/issues/new{{if .NewIssueChooseTemplate}}/choose{{end}}?milestone={{.MilestoneID}}">{{.locale.Tr "repo.issues.new"}}</a>