Fetch public servers and display them in a list for registration

For new users, it's not clear where and which servers to register if they don't have a pre-existing Mastodon account. While Tokodon supports registration, it still required you to know a public server URL beforehand.

Now a nice list of public servers (courtesy of joinmastodon.org) is displayed and you can choose from the list. You currently cannot filter apart from the domain, but that can be added later. It's still possible to enter a custom domain as well.