Anonymize user information for anonymous posts and topics

[cathyxfeiyang]

Modify topics/posts.js to process posts in order to anonymize user details

Effort: medium Dependencies: (https://github.com/CMU-313/spring24-nodebb-apple/issues/30)

Acceptance criteria:

• Unit testing to verify anonymized user information for anonymous posts and topics, with 90% coverage for added lines
• Manual testing to confirm that user information is not displayed for anonymous posts and topics