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>

modules/templates/helper.go

@@ -141,7 +141,7 @@ func NewFuncMap() template.FuncMap {
 		"ReactionToEmoji": reactionToEmoji,
 
 		// -----------------------------------------------------------------
-		// misc
+		// misc (TODO: move them to MiscUtils to avoid bloating the main func map)
 		"ShortSha":                 base.ShortSha,
 		"ActionContent2Commits":    ActionContent2Commits,
 		"IsMultilineCommitMessage": isMultilineCommitMessage,