I've also fixed another message: if a file to be downloaded is not
found, we say the entered provider isn't a LEAP provider. This is based
on the fact that if provider.json doesn't exist/contain valid
information, then the url isn't pointing to a LEAP provider.

We don't assume ca.crt is in /ca.crt anymore, but fetch the complete url
from provider.json.

We also signup against users.json file instead of simple "users", which
worked for *.bitmask.net domains.

Functionality copied to the Release build.

Removed the default danger_on = true in the debug build.

If you choose one and then tap "Switch provider" button, no duplicated providers are shown.

This return to "app" instead of "bitmask_android" is due to this reading: https://developer.android.com/sdk/installing/studio-build.html#projectStructure
I'll have to tweak the final apk name in build.gradle.