Add client-side validation for Album#released_on

By adding a `max` attribute with today's date as the value, we can
prevent the album form from being submitted with an invalid (i.e.
future) date.

While this isn't very pretty (because it relies on native browser HTML
form validation), for most users it should prevent the scenario of
losing multiple uploaded tracks if an invalid release date is set.

Fixes #154.

app/views/albums/_form.html.erb

@@ -16,7 +16,7 @@
         <path stroke-linecap="round" stroke-linejoin="round" d="M12 9v3.75m9-.75a9 9 0 11-18 0 9 9 0 0118 0zm-9 3.75h.008v.008H12v-.008z" />
       </svg>
     </span> <p>File must be in <strong>jpeg</strong> or <strong>png</strong> format.</p></div>
-  <%= form.date_field :released_on, class: 'w-full mb-3' %>
+  <%= form.date_field :released_on, max: Time.zone.today, class: 'w-full mb-3' %>
 
   <section id="tracks">
   <h2 class="text-2xl tracking-tight leading-10 font-extrabold my-3">Tracks</h2>