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 #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.

[danieldegroot2]

I thought I should mention the following, though they may not apply.

• Check if other applications already use this.
• Check with OWG (upstream, security), DWG (account permissions, abuse), and/or EWG (funding) if needed.

If neither applies, feel free to hide this comment as off-topic.

[simonpoole]

This is just a POC left open for to show that it -could- be done.

As actually merging it would not only require adding a facility to delete the account (there's currently no api for that, but that could be worked around), but assuming legal liability for the account deletion process, there is just no way that is going to happen.

See https://support.google.com/googleplay/android-developer/answer/10144311?hl=en "Account Deletion Requirement",