Always show letters on the usage page

Even if your service doesn’t send letters now, it might have done
previously.

The original reason for hiding letters was because it wasn’t a mature
feature. But now that it is, we should make it discoverable even for
existing teams. So that means not conditionally hiding it.

This is the same thing we do for emails/texts, where even if you switch
them off they still show up on the dashboard and usage page.

tests/app/main/views/test_dashboard.py

@@ -795,7 +795,7 @@ def test_usage_page(
     mock_get_usage.assert_called_once_with(SERVICE_ONE_ID, 2011)
     mock_get_free_sms_fragment_limit.assert_called_with(SERVICE_ONE_ID, 2011)
 
-    cols = page.find_all('div', {'class': 'column-half'})
+    cols = page.find_all('div', {'class': 'column-one-third'})
     nav = page.find('ul', {'class': 'pill', 'role': 'tablist'})
     nav_links = nav.find_all('a')
 
@@ -813,7 +813,7 @@ def test_usage_page(
     assert 'April' in table
     assert 'February' in table
     assert 'March' in table
-    assert '£15.84' in table
+    assert '£20.59' in table
     assert '140 free text messages' in table
     assert '£20.30' in table
     assert '1,230 text messages at 1.65p' in table