feat: use combo markdown editor for milestone description

- Use the combo markdown editor for the milestone description. The
milestone description is rendered in markdown, so it makes sense to use
a 'markdown-aware' editor. This also includes the option to use
monospace font.
- Resolves #5649

templates/repo/issue/milestone_new.tmpl

@@ -35,8 +35,15 @@
 					<input type="date" id="deadline" name="deadline" value="{{.deadline}}" placeholder="{{ctx.Locale.Tr "repo.issues.due_date_form"}}">
 				</div>
 				<div class="field">
-					<label>{{ctx.Locale.Tr "repo.milestones.desc"}}</label>
-					<textarea name="content">{{.content}}</textarea>
+					{{template "shared/combomarkdowneditor" (dict
+						"MarkdownPreviewUrl" (print .Repository.Link "/markup")
+						"MarkdownPreviewContext" .RepoLink
+						"TextareaName" "content"
+						"TextareaPlaceholder" (ctx.Locale.Tr "repo.milestones.desc")
+						"TextareaAriaLabel" (ctx.Locale.Tr "repo.milestones.desc")
+						"TextareaContent" .content
+						"EasyMDE" true
+					)}}
 				</div>
 				<div class="divider"></div>
 				<div class="tw-text-right">