Check for network access

[meisenzahl]

Fixes #127 Alternative approach to https://github.com/elementary/onboarding/pull/140

https://user-images.githubusercontent.com/10796736/137073621-eb121ca7-1c3a-4d2b-a2e8-5370d0500a42.mp4

@elementary/ux feel free to improve texts and layout 😅

[danirabbit]

I feel like launching full switchboard here is probably not a great idea since there's a lot of other things users could navigate to that don't make sense in this context

Ubiquity used to include a list of wireless networks natively in the installer for this step. I imagine it would be quite a bit more streamlined to do the same.

I also wonder if we shouldn't automatically go to the next step once we're connected?

Making a note that this is a pre-req for #35 And it makes sense to put this before the user/hostname step because of #120

[danirabbit]

Revamped this a little bit. Since we have the network indicator available, we can onboard users to its functionality instead of duplicating UI. Also be more specific about functionality that requires internet