Add backend social timetabling support

[rfquin]

Description 🧾

Context of PR, summary of changes, as well as any relevant technical decisions/motivations.
Adds social timetable support; namely, two individual tasks:

• Add an invitation code to the user schema
• Create a friendship module with the friendship model (Backend Rewrite - Add friend routes #1024)

Invitation Code

• See review comments

Friendship module & routes

Steps

• Add friendship model to prisma schema
• Create nest/ts files in module; e.g. controller / service / module / types
• Add REST function controller templates for required routes
  - Leave with "unimplemented!" Error
  - Take description of the service functions required
  - Full list at bottom of Description
• Implement service functions to yoink/set with prisma
• Replace the stubs in friendship.controller.ts with a real implementation

Testing

Replace this line with instructions on how to test your changes, as well as any relevant images for UI changes.

Checklist

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• 🎟️ To categorise release notes, the pull request should be labelled with at least one of these labels:
  • feature: for application feature improvement or new features
  • bug: fixing something that previously wasn't working
  • dev: development-side related changes
  • docker: updates to the Docker code
  • setup: changes to the setup/infrastructure of the codebase
  • test: updates to internal testing