Backend: Implement keyword-based search mechanism

[cindy-wg]

Task: Add a keyword-based search functionality to retrieve posts based on user-inputted keywords.
Estimated Effort: High
Dependencies: Search library, #27
Acceptance Criteria: Posts that match the user-entered keywords(s) are shown and edge cases such as having no matching keywords (and possibly keyword spelling mistakes by the user, etc.) are handled.

Milestone Update: Had to change from Sprint 1 to Sprint 2 because I underestimated the scope of this implementation and how long it would take to understand where this fits into the codebase.