Added in seeded fallback banners for users #1494

Closes #1494 (closed)

Summary

Adds a seeded default banner for a user

Steps

Log in, have a flick through https://feat-seeded-banner-1494.minds.io/discovery/search?q=&f=channels Example of account that has a banner: https://feat-seeded-banner-1494.minds.io/testacc/

Regression Scope

Limited to the banner endpoint, but:

Maybe a banner doesn't work? Maybe the set banner could replace it if the logic in the controller is flawed.