Move pagination utils into own module

This continues the pattern established in [1], just to chip away a
bit more at the random collection of stuff in utils/__init__.py.

[1]: https://github.com/alphagov/notifications-admin/pull/3923

tests/app/utils/test_pagination.py

@@ -0,0 +1,20 @@
+from app.utils.pagination import generate_next_dict, generate_previous_dict
+
+
+def test_generate_previous_dict(client):
+    ret = generate_previous_dict('main.view_jobs', 'foo', 2, {})
+    assert 'page=1' in ret['url']
+    assert ret['title'] == 'Previous page'
+    assert ret['label'] == 'page 1'
+
+
+def test_generate_next_dict(client):
+    ret = generate_next_dict('main.view_jobs', 'foo', 2, {})
+    assert 'page=3' in ret['url']
+    assert ret['title'] == 'Next page'
+    assert ret['label'] == 'page 3'
+
+
+def test_generate_previous_next_dict_adds_other_url_args(client):
+    ret = generate_next_dict('main.view_notifications', 'foo', 2, {'message_type': 'blah'})
+    assert 'notifications/blah' in ret['url']