Issue 148 self consistent requirements

Our build was failing: https://gitlab.developers.cam.ac.uk/uis/devops/uga/pool-applicant-document-management/-/jobs/503043 as the API to gspread has changed and we always install the latest from requirements. This non-deterministic behaviour is not acceptable.

A first attempt to solve this (by pinning our requirements to the same as geddit and making sure our test and docker requirements were also consistent) led to more complexity, and ultimately an unresolvable conflict, most likely because geddit uses a python3.7 image while claiming support for 3.6+. Instead, this was solved by adapting to the latest gspread API and importing Cell from gspread.cell instead of gspread.models.

To make sure that our requirements are always installable we must pin our package versions. To that end I have added pip-compile (from pip-tools) to our workflow.

closes: uis/devops/digital-admissions/synchronisation-service#148