feat: Revamp Referral Process

[samhwang]

Description

As already described in #205, at its current state, referrals are saved anonymously, which leads to a few issues. This referral aims to resolve that, but also added a few niceties.

• Update the Referral model to link to an owner userId that links to a User
• Update the /referral new command:
  • Update the response message to indicate who added the referral code
  • Add a dupe check logic to block a code if the user already added the code for said service
  • Make expiry_date an optional field. It will now default to 30 days from the added date.
• Update the /referral random message to also include the display name or the userid of the adder. This will help the end-user knows who to give rep when they use the code.

Motivation and Context

Resolves #205
Resolves #177

How Has This Been Tested?

• All existing and newly added unit tests passed
• Tested on the Bot Test server

What's next?

This PR has to stay in draft for the time being so the admins can truncate the referrals table before this change can be migrated.

Screenshots (if appropriate):

• Add new code
  
• Add new code with a defined expiry date
  
• Block adding service code if a code for said service already added by the user
  

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.