Add two-way messaging view

> Once an inbound message has been received, there should be a way to
> see the other messages in the system from the same service to the same
> number. Both in and outbound. Nice inbox/whatsapp stylee view or some
> such. This way the context of the reply is understood.
>
> Initially will only see the outbound template, not the actual message,
> but we’re going to change this for the rest (soon), so that you can
> always see the full message for all outbound.

app/templates/views/dashboard/inbox.html

@@ -25,7 +25,12 @@
       field_headings_visible=False
     ) %}
       {% call field() %}
-        <span class="file-list-filename" href="#">{{ item.user_number | format_phone_number_human_readable }}</span>
+        <a
+          class="file-list-filename"
+          href="{{ url_for('.conversation', service_id=current_service.id, notification_id=item.id) }}#n{{ item.id }}"
+        >
+          {{ item.user_number | format_phone_number_human_readable }}
+        </a>
         <span class="wide-left-hand-column">{{ item.content }}</span>
       {% endcall %}
       {% call field(align='right') %}