[BB-3173] Fix calculating vacations and commitments

[Agrendalath]

What it is

This contains the following changes:

1. Fixes calculating commitments and vacations with the assumption that a new sprint starts at midnight UTC.
2. Upgrades Python to 3.9 and Django to 3.1.3 and bumps dependencies (minor changes; unfortunately we cannot use PEP 585, because mypy does not support it yet).
3. Removes unused code from the cookiecutter template.

Testing instructions

1. Set .env and frontend/env.local (you can find them in a comment on the ticket).
2. Comment out these two lines. They're irrelevant to the scope and will only slow down the local environment.
3. Start the backend with docker-compose -f local.yml up --build.
4. Register new account and get the verification link with docker-compose -f local.yml logs django.
5. Run migrations with docker-compose -f local.yml exec django python manage.py migrate.
6. Navigate to frontend directory and run npm i && npm start.
7. Log in and navigate to cell's board.
8. Create an "off" calendar event for 4 edge cases described in the _get_vacation_for_day's docstring. Change your timezone here to a negative one to see how it impacts these cases.

[Agrendalath]

This is not our use case now, so I omitted this.

[0x29a]

👍

• I tested that all 8 scenarios from _get_vacation_for_day docstring work properly for team members from different time zones.
• I read through the code
• Includes documentation