Add a signup button for nextcloud

Krille

Maybe you don't know but nextcloud is not only a self-hosting service. It is as easy as dropbox and you can sign up on a public provider on this page: https://nextcloud.com/signup/

nextcloud.com automatically chooses a public provider in the near and you can easy set up your cloud. We could integrate this link as a "Sign up" button which opens a webview (like it does with the google account).

The overall user experience is way better if you are using a nextcloud. You can sync your calendar, in future your contacts too and it offers a lot of possibilities for app developers to backup app data in your nextcloud.

Other mobile operating systems like android and iOS are always using a clode provider by default to improve the user experience. With nextcloud we have a decentralized competition which we don't have to host by ourself. We only need to pick it up. What do you think?

@krille

I think Nextcloud really strengthens UT.