Update design and content of Tor VPN's onboarding dialog

There is a warning dialog that appears on first launch in our pre-alpha releases:

Click to expand

The dialog currently reads:

This app is highly experimental

Report bugs
Don't forget to report any bugs you find back to our developers.

But don't do anything risky
This is a pre-release version of the app, and is not safe to use for sensitive activities.

[Got it]

This dialog was designed to warn users who happened to find and install the pre-release version of the app that it is highly unstable, and we should revise the content or replace the dialog entirely in time for the open beta.