Warn family if demographic survey is absent

[mekline]

Description

How to reproduce

If you create a new account and click on the "Children Information" button to add a child without completing the demographic form, the "Find a Study" bar stays greyed out and the UI doesn't give any hints about why this might be.

Expected behavior

If the user has deviated from the expected workflow (go through forms one by one, saving and continuing to the next screen) everything is fine, but if you get off track it's not clear how to resume. There should be an indicator of some kind, probably for both the demographic & child, along the lines of "Add your demographic info to see studies you're eligible for", "Add at least one child to see studies you're eligible for".