Implement Rudimentary Access Control

Here we implement rudimentary access control by requiring users to
be logged in for some actions.
Associate Submissions with Users

Ultimately, we want submissions to be tied to the user that submitted
it. For that we are creating a migrations file with
`rails generate migration AddUserToSubmissions user:belongs_to`
and then `rake db:migrate`

Now that we have that association set up in the models, its easy for
us to set up the logic in the controller and views.