Allow multiple discord accounts for a single user

[ankith26]

Description

Currently if a user uses CAS to verify their discord account, that account gets
mapped with their CAS credentials and if any other alt account is used, again the
credentials need to be remapped.

It would be nice to add support for any discord alts users may have.

Tasks

• Current Database schema implemented in Mongo, need to be changed to accomodate multiple
  discord accounts
• Need to change the code-base such that it tries to query if an account is already linked and update it.
  Or else it creates a new one.

Testing

contact the maintainer to get access to test credentials