fix: return appropriate message on successful authentication login and logout.

[brarsanmol]

Summary

The authentication controller documentation is supposed to return a message when login/logout is successful it currently does not do this because of the variable being referred not existing. This PR brings the codebase up to date with the specifications of the documentation (https://docs.mchacks.ca/api/#api-Authentication-login).

Type of change

• [X] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] My changes generate no new warnings
• [ ] Listed change(s) in the Changelog
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes
• [X] I have made corresponding changes to the documentation
• [X] Any dependent changes have been merged and published in downstream modules