Load mentionValues asynchronously (#36739)

Eliminate a few database queries on all issue and pull request pages by
moving mention autocomplete data to async JSON endpoints fetched
on-demand when the user types `@`.

See https://github.com/go-gitea/gitea/pull/36739#issuecomment-3963184858
for the full table of affected pages.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/devtest/gitea-ui.tmpl

@@ -184,7 +184,7 @@
 
 	<div>
 		<h1>ComboMarkdownEditor</h1>
-		{{template "shared/combomarkdowneditor" dict "MarkdownPreviewContext" "/owner/path"}}
+		{{template "shared/combomarkdowneditor" dict "MarkdownEditorContext" (ctx.MiscUtils.MarkdownEditorGeneral nil)}}
 	</div>
 
 	<h1>Tailwind CSS Demo</h1>