Welcoming App support

[ty-everett]

When an app needs to convince new users to join the Metanet, conversion is extremely important. We need a system that creates cohesion, encouraging the user to successfully navigate the onboarding process by letting the welcoming app promote its content during sign-up.

This is achieved in two parts:

1. The welcoming app copies a JSON object to the user's clipboard containing relevant information like the app's name, icon, color scheme, and a "custom message" shown to the user. A "redirect URL" should also be included.
2. In this repository, when the Greeter page first loads, we read the clipboard and look for the JSON object.
3. If the object is found, the content is displayed during the sign-up process so the user knows where they're going.
4. Finally, after registration is complete, the user is taken to the "redirect URL" provided by the app, using window.open to launch the user's default browser.