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/main/views/platform_admin.py

@@ -27,12 +27,12 @@ from app.statistics_utils import (
     get_formatted_percentage,
     get_formatted_percentage_two_dp,
 )
-from app.utils import (
+from app.utils.csv import Spreadsheet
+from app.utils.pagination import (
     generate_next_dict,
     generate_previous_dict,
     get_page_from_request,
 )
-from app.utils.csv import Spreadsheet
 from app.utils.user import user_is_platform_admin
 
 COMPLAINT_THRESHOLD = 0.02