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

app/utils/pagination.py

@@ -0,0 +1,27 @@
+from flask import request, url_for
+
+
+def get_page_from_request():
+    if 'page' in request.args:
+        try:
+            return int(request.args['page'])
+        except ValueError:
+            return None
+    else:
+        return 1
+
+
+def generate_previous_dict(view, service_id, page, url_args=None):
+    return generate_previous_next_dict(view, service_id, page - 1, 'Previous page', url_args or {})
+
+
+def generate_next_dict(view, service_id, page, url_args=None):
+    return generate_previous_next_dict(view, service_id, page + 1, 'Next page', url_args or {})
+
+
+def generate_previous_next_dict(view, service_id, page, title, url_args):
+    return {
+        'url': url_for(view, service_id=service_id, page=page, **url_args),
+        'title': title,
+        'label': 'page {}'.format(page)
+    }