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.

routers/web/repo/patch.go

@@ -79,7 +79,7 @@ func NewDiffPatchPost(ctx *context.Context) {
 	// `message` will be both the summary and message combined
 	message := strings.TrimSpace(form.CommitSummary)
 	if len(message) == 0 {
-		message = ctx.Tr("repo.editor.patch")
+		message = ctx.Locale.TrString("repo.editor.patch")
 	}
 
 	form.CommitMessage = strings.TrimSpace(form.CommitMessage)