Add support for requesting a CAPTCHA during registration

[golf1052]

If a user gets blocked by a 402 error during registration this means that they need to complete a CAPTCHA to resolve the error. Signal uses https://signalcaptchas.org/registration/generate.html

See signalapp/Signal-Android@02b0800

[golf1052]

Sending CAPTCHA tokens to Signal works, the current problem I'm blocked by is that the URI generated by the Signal CAPTCHA page on https://signalcaptchas.org/registration/generate.html isn't a valid URI. This means when embedding the CAPTCHA webpage into the app I cannot handle any Navigation event correctly because the Windows APIs expect a valid URI. This also goes for URI protocol handling.

I've made a post on the Signal community forum. Hopefully Signal will consider making a change to their page. In the mean time I'll continue looking for a work around.

[golf1052]

I've figured out an annoying but working workaround. Though the C# Uri constructor fails to parse the signalcaptcha Uri, both a C++ or a Javascript UWP app can parse it fine (or at least don't throw an exception). With this knowledge here is the workaround.

1. Create a Javascript UWP app and register signalcaptcha as a protocol. The Javascript app is here. Then run the app to ensure the app is registered on the system to handle the signalcaptcha protocol.
2. Open a WebView in Signal-Windows and let the user solve the CAPTCHA. Once they do that the page will redirect to the signalcaptcha Uri which will then cause the system to prompt the user to open the Javascript CAPTCHA app.
3. When the Javascript app gets launched by the protocol, extract the token from the Uri and pass the token back to Signal-Windows. There are many ways to share data between UWP apps but I've opted to use LaunchUriAsync.
4. Since Signal-Windows is already open, when handling the protocol launch update the CAPTCHA token on the RegistrationPage.

The one major annoyance is that the user needs to install another app to get CAPTCHAs to work but I haven't found an in-app way of getting the signalcaptcha redirect to work yet.

[golf1052]

Support for this has been added in 2933a1e

[golf1052]

Signal Windows 0.3.0 is in the store.