Add the new alert button to previous alerts page

Feels like it should behave the same way as the current alerts page.

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

@@ -1,4 +1,5 @@
 {% from 'components/ajax-block.html' import ajax_block %}
+{% from "components/button/macro.njk" import govukButton %}
 
 {% extends "withnav_template.html" %}
 
@@ -12,4 +13,15 @@
 
   {% include('views/broadcast/partials/dashboard-table.html') %}
 
+  {% if current_user.has_permissions('send_messages') %}
+    <div class="js-stick-at-bottom-when-scrolling">
+      {{ govukButton({
+        "element": "a",
+        "text": "New alert",
+        "href": url_for('.new_broadcast', service_id=current_service.id),
+        "classes": "govuk-button--secondary"
+      }) }}
+    </div>
+  {% endif %}
+
 {% endblock %}

tests/app/main/views/test_broadcast.py

@@ -357,16 +357,20 @@ def test_broadcast_dashboard(
     )
 
 
+@pytest.mark.parametrize('endpoint', (
+    '.broadcast_dashboard', '.broadcast_dashboard_previous',
+))
 def test_broadcast_dashboard_does_not_have_button_for_view_only_user(
     client_request,
     service_one,
     active_user_view_permissions,
     mock_get_broadcast_messages,
+    endpoint,
 ):
     service_one['permissions'] += ['broadcast']
     client_request.login(active_user_view_permissions)
     page = client_request.get(
-        '.broadcast_dashboard',
+        endpoint,
         service_id=SERVICE_ONE_ID,
     )
     assert not page.select('a.govuk-button')
@@ -420,6 +424,15 @@ def test_previous_broadcasts_page(
         'Example template This is a test Broadcast yesterday at 2:20am England Scotland',
     ]
 
+    button = page.select_one(
+        '.js-stick-at-bottom-when-scrolling a.govuk-button.govuk-button--secondary'
+    )
+    assert normalize_spaces(button.text) == 'New alert'
+    assert button['href'] == url_for(
+        'main.new_broadcast',
+        service_id=SERVICE_ONE_ID,
+    )
+
 
 def test_new_broadcast_page(
     client_request,