feat: make code for referrals unique

[samhwang]

Description

This will introduce a DB migration to make the referral codes field unique. Although this may look simple, but the merge process will be difficult since it involves another script to dump out all of the referral code, check for duplicate and then re-insert into the table without disrupting the flow.

Motivation and Context

This will resolve #117

Screenshots (if appropriate):

Types of change

• 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.

[samhwang]

All referral codes has been exported, trimmed off the duplicates and re-inserted into Prod db.