Resolve "Fix logout" (1)

Closes #38 (closed)

What does this MR do (summary)?

Changes logout to POST so it actually works.

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.~~
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.