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/charset/escape_stream.go

@@ -173,7 +173,7 @@ func (e *escapeStreamer) ambiguousRune(r, c rune) error {
 		Val: "ambiguous-code-point",
 	}, html.Attribute{
 		Key: "data-tooltip-content",
-		Val: e.locale.Tr("repo.ambiguous_character", r, c),
+		Val: e.locale.TrString("repo.ambiguous_character", r, c),
 	}); err != nil {
 		return err
 	}