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.
- [x] Switch to iOS 8-style presentations (using `UIPresentationController`).
    - [x] Selectotron (posts page selector)
    - [x] Posts page settings
    - [x] Posts/PM page user actions and post actions
    - [x] Use `UIAlertController` for alerts and for action sheets.
- [x] Use auto layout-resizing table view cells...
    - [x] for the forum list.
    - [x] for thread lists.
    - [x] for PM list.
- [x] Switch to `WKWebView`...
    - [x] in Awful Browser.
    - [x] for profiles.
    - Since custom URL protocols don't work in `WKWebView`, I'm leaving posts, previews, and PMs with UIWebView for now.
- [x] Storyboard launch screen.
- [x] Use UIActivityViewController everywhere for links and images.
    - [x] Image preview.
    - [x] Long-pressing links.
    - [x] Awful Browser.
- [x] Ensure "Read Later" services remain represented in UIActivityViewController.
    - Either add UIActivity subclasses for them, or assume their apps will include extensions.
- [ ] [Native app-to-Web browser handoff](https://developer.apple.com/library/prerelease/ios/documentation/UserExperience/Conceptual/Handoff/AdoptingHandoff/AdoptingHandoff.html#//apple_ref/doc/uid/TP40014338-CH2-SW21)