Include Alpha and Nightly in MOZ_APP_DISPLAYNAME (and possibly in other places)

MOZ_APP_DISPLAYNAME is a variable that currently we set to Tor Browser and Mullvad Browser, but instead we could also include the channel for the alpha and nightly channel.

I've started adding the channel to the Windows installer, but changing also this value will make this value appear in other parts, starting from the macOS bundle name (which is highly desired, if I understand correctly) and in the properties of the binaries.

Mullvad Browser will need this to be set as a default browser, if we want the feature to be enabled in all channels without having the channels conflict. FWIW, Firefox does something similar (at least for the nightly channel).

It won't be shown in the browser, because translations are used instead (-brand-short-name).

Do we see any problem with this?

Should we update also the translations so that we see it also in the browser?