Add singular descriptions for libraries

This lets us write nice interface copy like ‘Choose a local authority
from the local authorities library’.

tests/app/broadcast_areas/test_broadcast_area.py

@@ -164,6 +164,6 @@ def test_repository_has_all_libraries():
 
     assert len(libraries) == 2
     assert [
-        'Countries',
-        'Local authorities',
-    ] == [name for _, name, _is_group in libraries]
+        ('Countries', 'country'),
+        ('Local authorities', 'local authority'),
+    ] == [(name, name_singular) for _, name, name_singular, _is_group in libraries]

tests/app/main/views/test_broadcast.py

@@ -320,6 +320,9 @@ def test_choose_broadcast_area_page(
         broadcast_message_id=fake_uuid,
         library_slug='ctry19',
     )
+    assert normalize_spaces(page.select_one('h1').text) == (
+        'Choose countries'
+    )
     assert [
         (
             choice.select_one('input')['value'],
@@ -347,6 +350,9 @@ def test_choose_broadcast_area_page_for_area_with_sub_areas(
         broadcast_message_id=fake_uuid,
         library_slug='wd20-lad20',
     )
+    assert normalize_spaces(page.select_one('h1').text) == (
+        'Choose a local authority'
+    )
     partial_url_for = partial(
         url_for,
         'main.choose_broadcast_sub_area',