Offer to connect to Planetary pubs after key restore

[mplorentz]

Currently we connect to the Planetary pubs if the user has not joined any others, even if the user opted not to "join the Planetary system" in onboarding (or by importing their key from another app). We should detect the case where the user's feed is empty and offer a one time connection to the Planetary pubs (or perhaps any pub they have an address for) so they can redownload their feed.