Refactor data model and syncing + implement NATS consumer

[FelixTJDietrich]

Motivation

We will need most of the GitHub entities and properties eventually. Introducing them later will be a higher cognitive load and will lead to some migration issues.

Description

• Refactor data model
  • Rename codereview module to gitprovider (maybe there is an even better name)
  • Add Issue
  • Change PullRequest to be subclass of Issue
  • Add Milestone
  • Add most of the relationships
• Introduce NATS consumer
  • Receive events via MessageHandlers that are automatically registered
  • Process pull_request payloads (TODO: process repos)
  • Process issues payloads
  • Process pull_request_review payloads
  • Process pull_request_review_comment payloads
  • Process issue_comment payloads
  • Process label payloads?
  • Process milestone payloads?
• Refactor syncing code for the future (prepares for GitLab integration, splits syncing code by entity, links entities)
  • Share code with webhook payloads to process entites
  • Link all relationships
• Fix client code (controllers, DTOs, etc.)

Screenshots (if applicable)

Checklist

General

• PR title is clear and descriptive
• PR description explains the purpose and changes
• Code follows project coding standards
• Self-review of the code has been done
• Changes have been tested locally
• Screenshots have been attached (if applicable)
• Documentation has been updated (if applicable)

Client (if applicable)

• UI changes look good on all screen sizes and browsers
• No console errors or warnings
• User experience and accessibility have been tested
• Added Storybook stories for new components
• Components follow design system guidelines (if applicable)

Server (if applicable)

• Code is performant and follows best practices
• No security vulnerabilities introduced
• Proper error handling has been implemented
• Added tests for new functionality
• Changes have been tested in different environments (if applicable)