Setting up PostgreSQL database and django-environ

[aaronjae22]

This PR sets up PostgreSQL as the database backend for the project and integrates django-environ to manage environment variables securely.

PostgreSQL Configuration:

• Added PostgreSQL as the database backend.
• Updated settings.py to load the database connection string from the .env file using django-environ.

Environment Variable Management:

• Introduced the django-environ package for secure and flexible management of environment variables.
• Added a .env file to store sensitive settings like SECRET_KEY, DEBUG, and the database connection string.

Examples:

Getting all objects linked to an Actor

Getting Alice's PortabilityOutbox

The PortabilityOutbox model aggregates all activities (Activity) performed by an Actor and related notes (Note). It relies on the relationships between these models to function properly, ensuring compliance with LOLA and ActivityPub requirements for account portability.

The PortabilityOutbox relies on the following models:

1. Actor: Represents the user who owns the activities and outbox.
2. Activity: Represents actions performed by the Actor (e.g., creating, liking, or announcing content).
3. Note: Represents content created or referenced in the activities.

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Closes #6
Closes #8
Closes #16