Address TODO: when Guardian > v1.1.0 is released

[indocomsoft]

Refactor to use on_refresh callback in the authentication code as well

[indocomsoft]

Blocked by #338, because Guardian 2.0 depends on Ecto 3

[indocomsoft]

For reference: https://github.com/source-academy/cadet/blob/master/lib/cadet/auth/guardian.ex#L36 and https://github.com/source-academy/cadet/blob/master/lib/cadet_web/controllers/auth_controller.ex#L62