Extract user utility code into own module

This provides more room for expansion, and reduces the amount of
arbitrary code in the __init__.py file for the new package.

app/main/views/platform_admin.py

@@ -32,8 +32,8 @@ from app.utils import (
     generate_next_dict,
     generate_previous_dict,
     get_page_from_request,
-    user_is_platform_admin,
 )
+from app.utils.user import user_is_platform_admin
 
 COMPLAINT_THRESHOLD = 0.02
 FAILURE_THRESHOLD = 3