first-class support for launching cloud from Android

[jab]

This is limited by hosting providers, who don't currently have sufficient APIs (for creating accounts and adding a billing method) which we could call from our own UI.

However, it is possible to:

• load a provider's own site in a WebView from within our app
• tell what page the WebView has loaded, and provide corresponding contextual hints (e.g. breadcrumbs, steppers)
• register an intent to intercept any email confirmation links that the user has to click
• poll in the background so that when the user clicks the link, if they open it in another browser rather than our app, display a notification to get the user to come back to our app
• inject JavaScript into the WebView to modify the provider's UI if necessary (e.g. optimizing DO's UI for mobile)
• detect the moment the user no longer needs to continue down the provider's own setup flow, and promptly hide the WebView and take them back into entirely our own UI, before they ever get prompted to create any droplets, VMs, or see anything else that could be confusing

[jab]

closing as dup of #2826