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/repo/wiki/new.tmpl

@@ -23,8 +23,7 @@
 			{{end}}
 			{{template "shared/combomarkdowneditor" (dict
 				"CustomInit" true
-				"MarkdownPreviewInRepo" $.Repository
-				"MarkdownPreviewMode" "wiki"
+				"MarkdownEditorContext" (ctx.MiscUtils.MarkdownEditorWiki $.Repository)
 				"TextareaName" "content"
 				"TextareaContent" $content
 				"TextareaPlaceholder" (ctx.Locale.Tr "repo.wiki.page_content")