Reimplement post detail view as UIPageViewController

[Physikbuddha]

The current solution to switch between individual posts by swiping left and right incorporates a UICollectionView which is good but not perfect for this kind of full screen presentation, as it comes with a number of drawbacks reported in #15, #17, #16, #22 and #25.

Instead, a fullscreen carousel navigation should be implemented using UIPageViewController: https://www.raywenderlich.com/5758454-uiscrollview-tutorial-getting-started#toc-anchor-013 https://developer.apple.com/documentation/uikit/uipageviewcontroller