Offer option to find and download Awala-compatible apps within the private gateways

[gnarea]

Describe the problem

Any user that installs an Awala Gateway is told that they also need an Awala-compatible app, yet we don't offer any pointers on how to get one, so we're basically relying on users finding out through other means.

Describe the solution you'd like

We should offer the option to install Letro and maybe one or two additional apps. If the Internet is available, the apps would be installed via the Google Play Store; otherwise, they'd be scheduled to be downloaded from the courier (at the next sync).

Recommending a Relaycorp app like Letro is inconsistent with the spirit of net neutrality, although technically not a violation IMO, so we'd have to amend our T&Cs to explicitly allow this just to be safe.

Describe alternatives you've considered

We could require users to install F-Droid or Butter, but that'd be yet another app that they'd need to install. Plus, how do they get F-Droid/Butter if they don't have access to the Internet?

Additional context

We should avoid encouraging users to sideload apps as it's too dangerous and Google refuse to make it safe.