Bump deployment target to iOS 13

[robinspeijer]

This PR bumps the deployment target to iOS 13, and implements the most deprecation warnings that came with that change. Most important ones:

• We used a deprecated background fetch API. This PR will use the BackgroundTasks framework instead.
• Keyed archiver deprecated non-securecoding and non-generic coding methods. Updated our implementation in Settings and ImageUploader controllers
• Network activity indicator is deprecated in iOS 13, without replacement
• UIWebView is now deprecated. Our webviewcontroller now uses WKWebView
• kSecAttrAccessibleAlways is deprecated. I've replaced it with kSecAttrAccessibleAfterFirstUnlock