nav: Simplify types on navigation props.

This form is shorter, and also I think closer to the truth.

On the other hand, as the fixmes show, these types actually cause
the type-checker to give errors when we use these components in
`AppNavigator`!  In fact we'd need fixmes on more of them... if the
react-redux libdef didn't defeat type-checking at `connect` calls.

I think two things are going on in the fixmes:
 * The react-navigation libdef doesn't have the right covariance:
   it ends up wanting us to put more properties into the argument
   to NavigationScreenProp.
 * The libdef doesn't want to guarantee that anything in particular
   goes into `params` -- in which it is entirely correct!  That is
   an unchecked assumption we make, coupling `navActions` with the
   respective screen components.

src/chat/ChatScreen.js

@@ -10,13 +10,7 @@ import Chat from '../chat/Chat';
 import ChatNavBar from '../nav/ChatNavBar';
 
 type Props = {|
-  navigation: NavigationScreenProp<*> & {
-    state: {
-      params: {
-        narrow: Narrow,
-      },
-    },
-  },
+  navigation: NavigationScreenProp<{ params: {| narrow: Narrow |} }>,
 |};
 
 const styles = StyleSheet.create({

src/emoji/EmojiPickerScreen.js

@@ -19,13 +19,7 @@ type Props = {|
   activeImageEmojiByName: RealmEmojiById,
   auth: Auth,
   dispatch: Dispatch,
-  navigation: NavigationScreenProp<*> & {
-    state: {
-      params: {
-        messageId: number,
-      },
-    },
-  },
+  navigation: NavigationScreenProp<{ params: {| messageId: number |} }>,
 |};
 
 type State = {|

src/lightbox/LightboxScreen.js

@@ -18,14 +18,7 @@ const styles = StyleSheet.create({
 });
 
 type Props = {|
-  navigation: NavigationScreenProp<*> & {
-    state: {
-      params: {
-        src: string,
-        message: Message,
-      },
-    },
-  },
+  navigation: NavigationScreenProp<{ params: {| src: string, message: Message |} }>,
 |};
 
 export default class LightboxScreen extends PureComponent<Props> {

src/nav/AppNavigator.js

@@ -41,6 +41,7 @@ export default StackNavigator(
     'account-details': { screen: AccountDetailsScreen },
     'group-details': { screen: GroupDetailsScreen },
     auth: { screen: AuthScreen },
+    // $FlowFixMe react-navigation types :-/
     chat: { screen: ChatScreen },
     dev: { screen: DevAuthScreen },
     'emoji-picker': { screen: EmojiPickerScreen },
@@ -53,6 +54,7 @@ export default StackNavigator(
     users: { screen: UsersScreen },
     settings: { screen: SettingsScreen },
     language: { screen: LanguageScreen },
+    // $FlowFixMe react-navigation types :-/
     lightbox: { screen: LightboxScreen },
     group: { screen: CreateGroupScreen },
     'invite-users': { screen: InviteUsersScreen },

src/start/AuthScreen.js

@@ -20,13 +20,7 @@ import { loginSuccess, navigateToDev, navigateToPassword } from '../actions';
 type Props = {|
   dispatch: Dispatch,
   realm: string,
-  navigation: NavigationScreenProp<*> & {
-    state: {
-      params: {
-        serverSettings: ApiResponseServerSettings,
-      },
-    },
-  },
+  navigation: NavigationScreenProp<{ params: {| serverSettings: ApiResponseServerSettings |} }>,
 |};
 
 let otp = '';