Refactor/rename user database pk

[kiritowu]

Rationale:
As discussed earlier, current database schema does not correctly reflect the data stored in the database. In fact, in practice, we have been using email to identify user, but the database recorded it as UserName. This PR is to address that problem.

Changes:

• Apply supabase migration to rename column UserName -> email and email_address is username.
• Updated Frontend and Backend to reflect to latest schema. A quick run-through of major feature shows the code is working as intended.

[elliotwutingfeng]

LGTM