meson: Use meson.project_name() instead of app_id for pots

Ubuntu's dh_translations tool for its language packs assumes that the translation templates match the project id. Let's follow that pattern.

Before, the translation templates were named polari.pot and help-org.gnome.Polari.pot

Now they are named polari.pot and help-polari.pot

See comment 21 on https://launchpad.net/bugs/1762889