Awful 3.0: Embrace iOS 8

Let's delete all the custom crap we can! (Copy of prematurely merged #302)

- [x] Switch to `UISplitViewController` and `UITabBarController` on all devices.
- [ ] Switch to iOS 8-style presentations (using `UIPresentationController`).
    - [x] Selectotron (posts page selector)
    - [x] Posts page settings
    - [ ] Posts/PM page user actions and post actions
    - [ ] Use `UIAlertController` for alerts and for action sheets.
- [ ] Use auto layout-resizing table view cells for forums and threads.
- [ ] Switch to `WKWebView`.