Remove extra challenge box padding. Closes #34

[slashnick]

From base.html:

<div class="container">
  {%- block body %}{% end block %}
</div>

From challenge.html:

{% block body %}
  <div class="container"> ... </div>
{%- endblock %}

Markup in challenge.html was placed in a doubly-nested container div. This div adds 15px of padding for when the screen size is small, but this shifts the whole container right by 15px if it's in another fixed-width div. Since it was just in another container, we got 15px of unnecessary padding.