(Frontend) Create a "Save to Favorites" button in the topic page

[tu2463]

• In the category page and post detail page, add a star button next to each post to save it to favorites.
• Clicking the star button will label the post as saved or unsaved in the backend and highlight the button UI.

Category page:
Post detail page:

Estimated effort: 1h
Actual effort: 8h
Dependencies: none
Acceptance criteria:

• Click the button to save a post. Confirm that the post is labeled as saved in the backend and that the button's UI has changed in the frontend.
• Click the button again to unsave the post. Check the same thing in the backend and frontend.