Convert frontend display to be just UTC (#540)

This changeset converts the display of dates and times to be just UTC to match the recent changes in the backend.  This unwinds a bit of work that was done previously and allows us to start with a clean slate in how we want to approach displaying dates and times going forward. It also adds a bit of explanatory text to help users.

Signed-off-by: Carlo Costino <carlo.costino@gsa.gov>
Co-authored-by: stvnrlly <steven.reilly@gsa.gov>

app/templates/views/message-status.html

@@ -12,6 +12,12 @@
   <p>Notify’s real-time dashboard lets you check the status of any message.</p>
   <p>For <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for("main.security") }}">security</a>, this information is only available for seven days after a message has been sent. You can download a report, including a list of sent messages, for your own records.</p>
   <p>This page describes the statuses you’ll see when you’re signed in to Notify.</p>
+
+  <h2 id="text-message-timezones" class="heading-medium">A note about timezones</h2>
+
+  <p>Notify currently uses UTC (Coordinated Universal Time) for managing and displaying all dates and times in the system.  This means that when you're scheduling messages to send later, or are looking at the time a message was sent, it will be in the UTC timezone.</p>
+
+  <p>You can refer to <a href="https://en.wikipedia.org/wiki/Time_in_the_United_States#United_States_and_regional_time_zones">this chart for help</a> with converting UTC times to your local timezone.</p>
   <!-- <p>If you’re using the Notify API, read our <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.documentation') }}">documentation</a> for a list of API statuses.<p> 
 
   <h2 id="email-statuses" class="heading-medium">Emails</h2>

app/templates/views/notifications.html

@@ -61,6 +61,11 @@
        
       Data available for {{ partials.service_data_retention_days }} days
     </p>
+    <p class="font-body-sm">
+      <a href="https://en.wikipedia.org/wiki/Time_in_the_United_States#United_States_and_regional_time_zones">View timezone chart</a>
+      &emsp;
+      For help with understanding and converting UTC times
+    </p>
   {% endif %}
 
   {{ ajax_block(

app/templates/views/notifications/notification.html

@@ -35,7 +35,7 @@
       {% elif created_by %}
         by {{ created_by.name }}
       {% endif %}
-      {{ created_at|format_datetime_human }}
+      {{ created_at|format_datetime_human }} UTC
     </p>
 
     <div class="">