Update source interface message when disabled

See #4305 (comment)

securedrop/source_templates/disabled.html

@@ -1,7 +1,7 @@
 {% extends "base.html" %}
 {% block body %}
-<h1>{{ gettext('Submissions Disabled') }}</h1>
+<h1>{{ gettext("We're sorry, our SecureDrop is currently offline.") }}</h1>
 
-<p>{{ gettext('This SecureDrop instance is currently unavailable. Please check back later.') }}</p>
+<p>{{ gettext("Please try again later. Check our website for more information.") }}</p>
 
 {% endblock %}

securedrop/tests/test_source.py

@@ -729,7 +729,7 @@ def test_source_disabled_when_trusty_is_eol(config, source_app):
             resp = app.get(url_for(endpoint))
             assert resp.status_code == 200
             text = resp.data.decode('utf-8')
-            assert 'Submissions Disabled' in text
+            assert "We're sorry, our SecureDrop is currently offline." in text
 
 
 @mock.patch('platform.linux_distribution', side_effect=fake_trusty)
@@ -741,7 +741,7 @@ def test_source_not_disabled_before_trusty_eol(config, source_app):
             resp = app.get(url_for(endpoint), follow_redirects=True)
             assert resp.status_code == 200
             text = resp.data.decode('utf-8')
-            assert 'Submissions Disabled' not in text
+            assert "We're sorry, our SecureDrop is currently offline." not in text
 
 
 @mock.patch('platform.linux_distribution', side_effect=fake_xenial)
@@ -753,4 +753,4 @@ def test_source_not_disabled_xenial(config, source_app):
             resp = app.get(url_for(endpoint), follow_redirects=True)
             assert resp.status_code == 200
             text = resp.data.decode('utf-8')
-            assert 'Submissions Disabled' not in text
+            assert "We're sorry, our SecureDrop is currently offline." not in text