(Frontend and Backend) View saved to favorites posts under the same category

[tu2463]

Frontend: Clicking the Favorites button will only show saved posts in this category.
Backend: Clicking the Favorites button will retrieve all saved posts in this category.
This functionality will work like filtering by tags.

Estimated effort: 8h
Dependencies: #1 #31 #30
Acceptance criteria:

1. On Category 1 page, click the button next to a post to save it to favorites.
2. Open another post under Category 1 and click the button on the post details page to save it to favorites.
3. On Category 1 page, click the "Favorites" button on the menu bar. All saved to favorites posts under Category 1 should be shown.
4. Go to Category 2 page to save more posts and click the "Favorites" button. All saved to favorites posts under Category 2 should be shown.
5. Unsave some posts by clicking the buttons on both the category page and the post details page.
6. Click the "Favorites" button on the category page. It should not show the unsaved posts.