Remember User on same Computer / Device

[JonathanMH]

Users should be remembered on the same device, without having to sign up.

We propose to use localStorage for remembering a user by a random token or a token from the API that previously has not been seen.

If a user returns, they should be allowed to annotate the screenshot they created themselves.

[zkwsk]

I think this one should be covered under this, but we need to get back and test after it has been merged: #169