[User story] Search option

[alexkisielewicz]

As a user, I can search for posts with keywords, so that I can find the posts by title, username, or category they have been posted in.

Acceptance criteria:

• A search bar is provided where users can enter keywords to search for relevant posts.
• The search functionality returns posts that match the entered keywords (title, username, and category filters are in use).
• Feedback is provided in the UI if there are no results.
• Search request to the back-end is delayed after a keystroke so there are no unnecessary requests.