As a user with no interests, I would like to be invited to subscribe to some interests

Migrated from [trello card](https://trello.com/c/A2IJY487/5-when-i-am-not-subscribed-to-any-interests-or-only-everyone-i-should-see-text-inviting-me-to-select-some-interests).

If a user is not subscribed to any interests, it may be helpful to let them know they should select some. This could be a one-time banner or other notification.