Add brandProductName to torbutton brand localization files

brandProductName was already present in ESR68, but we did not update torbutton brand.dtd or brand.properties files. For 68, this does not break the browser, since brandProductName is not used as an xml entity (DTD), and the .properties usage is in a path we probably do not hit.

Currently, this is string is used in several additional places, and starting Firefox 70 it's used as a DTD entity, which prevents the browser from starting if it's not defined.