Added Sortable to families and tasks.

Implements #246.

After running into weird positioning issues with jquery-ui I ended up implementing this using Rubaxa Sortable. Browser support is all modern browsers and IE10+.
I had forgotten until now, but on the `checkin-kiosk-react` branch, I made a Reorder concern to be used by several models: https://github.com/churchio/onebody/blob/checkin-kiosk-react/app/models/concerns/reorder.rb

You're welcome to use that, or to use acts-as-list, or whatever. I just would like to share the code amongst all the models.

As for people sequence, the attribute is on the Person model -- it's just the method that exists on the Family.