SwiftUI Settings redux

[nolanw]

Picked up #1152 and ran with it:

• Move the Settings and Acknowledgements screens to SwiftUI.
  • Fix the app icon selector.
• Replace Sourcery with a centralized list of settings, conveniences for using settings within SwiftUI, and add dependency Foil for observability outside SwiftUI.
• Experiment with moving UI bits into packages.
  • AwfulExtensions for convenience bits that know nothing about Awful specifically.
  • AwfulModelTypes for some tagged string identifiers, hopefully we don't confuse e.g. forum IDs and thread IDs between public interfaces.
  • AwfulSettings and AwfulTheming are shared UI code, will probably get imported a lot.
  • AwfulSettingsUI is (more or less) the Settings tab.
  • This is mostly for faster builds when using previews. A fringe benefit is making the internal access level relevant.
  • Lots more imports needed when making a new view/screen. Certainly room to fiddle, but let's get a few more reps in first.
• Add some SwiftUI affordances for theming. Lots more to do here.
• No more Objective-C in the Awful target. Vendored Objective-C code is quarantined in dedicated packages.