Add Thunderbird for Android Beta links to downloads page

[kewisch]

https://stage.thunderbird.net/thunderbird/all/

We have beta and release links for Desktop, we should have the same for mobile.

Campaign links can be the same but with net.thunderbird.android.beta as the package id

This doesn't need to be ready by the release.

[MelissaAutumn]

Is something like this acceptable?

[kewisch]

Ideally I think we should have consistency with desktop where you can select the channel

[MelissaAutumn]

I'll bring this up in the next website meeting!