Show inbound messages on the dashboard

This commit adds two things:

a section on the dashboard to show how many inbound messages the
service has received in the last 7 days, and how recently an inbound
message has been received
---

Doesn’t show the contents of any messages, just like how the rest of the
dashboard is an aggregation, never individual messages.

a page to show all the inbound messages the service has received in
the last 7 days
---

This shows the first line of the message. Eventually this will link
through to a ‘conversation’ page, where a service can see all the
messages it’s received from a given phone number.

app/templates/views/dashboard/inbox.html

@@ -0,0 +1,43 @@
+{% from "components/table.html" import list_table, field, hidden_field_heading, right_aligned_field_heading, row_heading %}
+{% from "components/message-count-label.html" import message_count_label %}
+
+{% extends "withnav_template.html" %}
+
+{% block service_page_title %}
+  Inbox
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  <h1 class="heading-large">
+    Received text messages
+  </h1>
+  <div>
+    {% call(item, row_number) list_table(
+      messages,
+      caption="Inbox",
+      caption_visible=False,
+      empty_message='When users text your service’s phone number ({}) you’ll see the messages here'.format(current_service.sms_sender),
+      field_headings=[
+        'From',
+        'First two lines of message'
+      ],
+      field_headings_visible=False
+    ) %}
+      {% call field() %}
+        <span class="file-list-filename" href="#">{{ item.user_number }}</span>
+        <span class="wide-left-hand-column">{{ item.content }}</span>
+      {% endcall %}
+      {% call field(align='right') %}
+        <span class="file-list-hint align-with-message-body">
+          {{ item.created_at | format_delta }}
+        </span>
+      {% endcall %}
+    {% endcall %}
+    {% if messages %}
+      <p class="table-show-more-link">
+        Showing all {{ messages | length }} messages
+      </p>
+    {% endif %}
+  </div>
+{% endblock %}