Move "Today" heading to left column on platform admin page

When we make the numbers on this page more filterable the date range will be
one of the options to change, so it makes sense to move it to the side now
instead of leaving it above the big numbers.

app/templates/views/platform-admin.html

@@ -99,6 +99,7 @@
     <div class="column-one-third">
       <nav class="navigation">
         <ul>
+          <li>Showing stats for today</li>
           {% if include_from_test_key %}
             <li>Including test key (<a href="{{ url_for('.platform_admin', include_from_test_key=False) }}">change</a>)</li>
           {% else %}
@@ -108,7 +109,6 @@
       </nav>
     </div>
     <main role="main" class="column-two-thirds column-main">
-      <h2 class='heading-medium'>Today</h2>
       <div class="grid-row bottom-gutter">
         <div class="column-half">
           {{ big_number_with_status(