Check ggt.gaa.st domain verification

[Wilm0r]

See https://github.com/Wilm0r/giggity/pull/387

Looks like the ggt.gaa.st deeplinks aren't always working properly, for example not when the user has installed Giggity from F-Droid. A quick read of https://developer.android.com/training/app-links/verify-android-applinks shows two possible issues:

• https://ggt.gaa.st/.well-known/assetlinks.json only has my signing key, I may have to add the signing key used for F-Droid builds?
• Possibly I may have to ask the user to complete domain verification? Not great since I think not people actually use this feature so proactively always checking this at startup may be unnecessary. Maybe best to hide it in the settings menu somewhere with a hint on the barebone http://ggt.gaa.st/ page for users who accidentally get there.