You could have a look at EteBase [1], which was recently featured on HN. I haven't tried it myself, but I'm looking to build my applications in a similiar style.
Etebase didn't exist earlier this year when I wanted to build Portabella, but I think it's a great option for many platforms.
In that sense Firebase, Parse, deepstream.io all handle that problem as well.
What you want though is to operate without seeing your customers data, much more ethical in that you'll never be tempted to sell your users data