Site messages 'tag', enhancements for translations (#3718)

* site_messages 'tag' + mark translatable strings

- Simpler way to prevent duplicate messages of the same type
- Mark some left-out strings for translation.

* ngettext (pluralization support)

- Add comments for translators (if needed) by starting a comment line in a file with "TRANSLATORS:".
Not sure if Crowdin would recognize that as context. Python gettext doesn't support the context function (tornado's gettext does, but I need to look into this more).