Fix emoji replacements, make emoji images consistent (#12567)

- Fix emoji not being replaced in issue title change text
- Make the image attributes consistent, add alt, remove align

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

modules/markup/html.go

@@ -481,6 +481,7 @@ func createCustomEmoji(alias, class string) *html.Node {
 		Attr:     []html.Attribute{},
 	}
 	if class != "" {
+		img.Attr = append(img.Attr, html.Attribute{Key: "alt", Val: fmt.Sprintf(`:%s:`, alias)})
 		img.Attr = append(img.Attr, html.Attribute{Key: "src", Val: fmt.Sprintf(`%s/img/emoji/%s.png`, setting.StaticURLPrefix, alias)})
 	}