Reconcile not signed up user's screenshots on signup

[JonathanMH]

We need to make sure that the screenshots created by a user before their signup, remains theirs to edit and delete when they create a user account / register.

cc: @bytewiz Do you think it's possible to get an anonymous token through firebase auth that we later can assign an email address or a facebook user to?

This issue depends on #83

[bytewiz]

@JonathanMH Yes, that is possible 👍

[zkwsk]

Note that this one is low priority. We need to get the core functionality running first.