Make a separate page for previous alerts

Previous alerts are much less important than ones that are live or
waiting for approval.

Therefore we can make the dashboard more focused by moving previous
alerts to their own page.

app/templates/main_nav.html

@@ -6,6 +6,7 @@
   {% if current_user.has_permissions() %}
     {% if current_service.has_permission('broadcast') %}
       <li><a class="govuk-link govuk-link--no-visited-state{{ main_navigation.is_selected('dashboard') }}" href="{{ url_for('.broadcast_dashboard', service_id=current_service.id) }}">Dashboard</a></li>
+      <li><a class="govuk-link govuk-link--no-visited-state{{ main_navigation.is_selected('previous_broadcasts') }}" href="{{ url_for('.broadcast_dashboard_previous', service_id=current_service.id) }}">Previous alerts</a></li>
     {% elif current_user.has_permissions('view_activity') %}
       <li><a class="govuk-link govuk-link--no-visited-state{{ main_navigation.is_selected('dashboard') }}" href="{{ url_for('.service_dashboard', service_id=current_service.id) }}">Dashboard</a></li>
     {% endif %}

app/templates/views/broadcast/previous-broadcasts.html

@@ -0,0 +1,15 @@
+{% from 'components/ajax-block.html' import ajax_block %}
+
+{% extends "withnav_template.html" %}
+
+{% block service_page_title %}
+  Previous alerts
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  <h1 class="heading-medium govuk-!-margin-bottom-2">Previous alerts</h1>
+
+  {% include('views/broadcast/partials/dashboard-table.html') %}
+
+{% endblock %}