add logout route + tests

[juancwu]

Description

A logout route that unsets the refresh token cookie.

Linked Issues

• [BE Feat] Create Logout authentication endpoint #182

Testing

• add new test for the logout route in tests/server_test.go

Checklist

Before opening this PR, make sure the PR:

• Has an assignee or group of assignees.
• Has a reviewer or a group of reviewers.
• Is labelled properly.
• Has the SPUR project assigned to it.
• Has an assigned milestone.

Additionally, make sure that:

• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.

[aidantrabs]

lgtm