Fix off-by-one dates on provider history

We chunk history entries by their YYYY-MM-DD date representation to
display them day-by-day.

However trying to convert a YYYY-MM-DD string into a timestamp is
unpredictable, and was causing our dates to be one day out (probably
because of midnight/daylight savings/general datetime horrors).

This commit changes the code to do the same thing as the history page,
which is look at the `updated_at` field on the first entry to get a
datetime object and from that the formatted date we show in the headings
on the page.

app/templates/views/providers/edit-sms-provider-ratio.html

@@ -51,7 +51,7 @@
   {% for day, versions in versions %}
     <h2 class="heading-small top-gutter">
       {% if day %}
-        {{ day|format_date_human|title }}
+        {{ versions[0]['updated_at']|format_date_human|title }}
       {% else %}
         Start
       {% endif %}