webview: Replace UIWebView with WKWebView, on iOS.
webview: Replace UIWebView with WKWebView, on iOS.

React Native 0.57+ supports the new `WKWebView` iOS component in
`WebView` instead of the legacy `UIWebView`.

The UIWebView component is deprecated since iOS 12:
  https://developer.apple.com/documentation/uikit/uiwebview

WKWebView is supported on iOS 8 and newer so that is not a concern.
  https://developer.apple.com/documentation/webkit/wkwebview

An article on the `WKWebView` support in React Native here:
  http://facebook.github.io/react-native/blog/2018/08/27/wkwebview

The change code-wise is trivial, I have tested it on an emulator
and as expected everything seems to be working correctly.

We should do an extended TestFlight testing just in case.
To reiterate, this is a iOS-specific change.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile