Add function to start OSM signup in WebView

[simonpoole]

This starts the signup process in a webview and then captures the link from the confirmation e-mail to open it in the app. Pressing the "Start mapping" button then terminates the webview and leaves the user in Vespucci.

Would fix https://github.com/MarcusWolschon/osmeditor4android/issues/1506 if merged, but that wont happen in a month of Sundays for obvious reasons.

If the world stood still and it is merged. it would need, besides a test, a bit more refinement, for example immediately authorizing the app via OAuth 2 and then returning to the app.

[simonpoole]

Note if we ever merge this, googles play store rules requires that we allow account deletion from the app.