Replace fixed delay when performing Django DB migration

There is a sleep 8 to allow time for cloud_sql_proxy to create the socket for accessing the DB. It should be replaced with a check for the existence of the socket instead.

!34 (merged) implements this change (https://gitlab.developers.cam.ac.uk/uis/devops/digital-admissions/albatross/albatross-deployment/-/merge_requests/2 is where the issue was first spotted).