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/diff/box.tmpl

@@ -227,8 +227,7 @@
 				<div class="field">
 				{{template "shared/combomarkdowneditor" (dict
 					"CustomInit" true
-					"MarkdownPreviewInRepo" $.Repository
-					"MarkdownPreviewMode" "comment"
+					"MarkdownEditorContext" (ctx.MiscUtils.MarkdownEditorComment $.Repository)
 					"TextareaName" "content"
 					"DropzoneParentContainer" ".ui.form"
 				)}}

templates/repo/diff/comment_form.tmpl

@@ -1,4 +1,4 @@
-{{if and $.root.SignedUserID (not $.Repository.IsArchived)}}
+{{if and ctx.RootData.SignedUserID (not ctx.RootData.Repository.IsArchived)}}
 	<form class="ui form {{if $.hidden}}tw-hidden comment-form{{end}}" action="{{$.root.Issue.Link}}/files/reviews/comments" method="post">
 		<input type="hidden" name="origin" value="{{if $.root.PageIsPullFiles}}diff{{else}}timeline{{end}}">
 		<input type="hidden" name="latest_commit_id" value="{{$.root.AfterCommitID}}">
@@ -11,8 +11,7 @@
 		<div class="field">
 		{{template "shared/combomarkdowneditor" (dict
 			"CustomInit" true
-			"MarkdownPreviewInRepo" $.root.Repository
-			"MarkdownPreviewMode" "comment"
+			"MarkdownEditorContext" (ctx.MiscUtils.MarkdownEditorComment ctx.RootData.Repository)
 			"TextareaName" "content"
 			"TextareaPlaceholder" (ctx.Locale.Tr "repo.diff.comment.placeholder")
 			"DropzoneParentContainer" "form"

templates/repo/diff/new_review.tmpl

@@ -19,8 +19,7 @@
 			</div>
 			<div class="field">
 				{{template "shared/combomarkdowneditor" (dict
-					"MarkdownPreviewInRepo" $.Repository
-					"MarkdownPreviewMode" "comment"
+					"MarkdownEditorContext" (ctx.MiscUtils.MarkdownEditorComment $.Repository)
 					"TextareaName" "content"
 					"TextareaPlaceholder" (ctx.Locale.Tr "repo.diff.review.placeholder")
 					"DropzoneParentContainer" "form"