Make page titles rendered via user_nav or list_nav localizable.

If one runs a non-English Postorius site, one currently sees that the titles (head lines) of several pages are not translated into the sites language (here: German). The cause of this is that the via nav_helpers.py provided inclusion tags list_nav and user_nav support being localized. With this MR, I'd like to propose a fix/workaround for this, so that also the titles of those configuration pages in postorius appear in the sites locale.