Add Substitution model

[pniedzwiedzinski]

Substitution model would have these properties:

• lesson number
• absent teacher
• class
• subject name
• classroom
• substitute teacher
• date

[pniedzwiedzinski]

In this, we will need to add Postgres connection to settings.py and container configuration to docker-compose

[pniedzwiedzinski]

I edited the original comment since I forgot to include date field which will be crucial in future releases.