WIP: added a button that will enable users to display all entries, not just unread ones. It's still non-functional.
Horizontally center buttons inside their divs, so they are more evenly spaced.
When the user clicks on the "show unread entries" button, all entries for the currently selected feed are shown. Read and unread entries have different CSS classes (with green and grey color, respectively) so the user can clearly distinguish them.