Refactor locale&string&template related code (#29165)

Clarify when "string" should be used (and be escaped), and when
"template.HTML" should be used (no need to escape)

And help PRs like  #29059 , to render the error messages correctly.

modules/markup/html.go

@@ -804,7 +804,7 @@ func fullIssuePatternProcessor(ctx *RenderContext, node *html.Node) {
 		// indicate that in the text by appending (comment)
 		if m[4] != -1 && m[5] != -1 {
 			if locale, ok := ctx.Ctx.Value(translation.ContextKey).(translation.Locale); ok {
-				text += " " + locale.Tr("repo.from_comment")
+				text += " " + locale.TrString("repo.from_comment")
 			} else {
 				text += " (comment)"
 			}