Refactor form input styles

* Fix issue where button is rendered taller in flexbox, due to default
  margin on input elements
* Refactor/improve all login and signup views

app/views/signup/index.html.erb

@@ -11,6 +11,6 @@
   </p>
   <p class="mt-12">
     <%= link_to "Get started", signup_steps_path(1),
-          class: "btn-md btn-blue block w-full md:inline-block sm:w-auto" %>
+          class: "btn-md btn-blue block w-full" %>
   </p>
 <% end %>