flow: Add fixmes for upcoming upgrade.

When we upgrade Flow to v0.92 in tandem with RN to v0.59,
we'll start getting errors here.  Add fixmes for them.

Author: gnprice

src/main/MainTabs.js

@@ -17,6 +17,7 @@ import ProfileCard from '../account-info/ProfileCard';
 export default TabNavigator(
   {
     home: {
+      // $FlowFixMe react-navigation types are twisty and seem wrong
       screen: HomeTab,
       navigationOptions: {
         tabBarLabel: 'Home',
@@ -35,6 +36,7 @@ export default TabNavigator(
       },
     },
     conversations: {
+      // $FlowFixMe react-navigation types are twisty and seem wrong
       screen: PmConversationsCard,
       navigationOptions: {
         tabBarLabel: 'Conversations',
@@ -44,6 +46,7 @@ export default TabNavigator(
       },
     },
     settings: {
+      // $FlowFixMe react-navigation types are twisty and seem wrong
       screen: SettingsCard,
       navigationOptions: {
         tabBarLabel: 'Settings',
@@ -53,6 +56,7 @@ export default TabNavigator(
       },
     },
     profile: {
+      // $FlowFixMe react-navigation types are twisty and seem wrong
       screen: ProfileCard,
       navigationOptions: {
         tabBarLabel: 'Profile',

src/main/StreamTabs.js

@@ -19,6 +19,7 @@ const styles = StyleSheet.create({
 export default TabNavigator(
   {
     subscribed: {
+      // $FlowFixMe react-navigation types are twisty and seem wrong
       screen: SubscriptionsCard,
       navigationOptions: {
         tabBarLabel: (props: TabNavigationOptionsPropsType) => (
@@ -29,6 +30,7 @@ export default TabNavigator(
       },
     },
     allStreams: {
+      // $FlowFixMe react-navigation types are twisty and seem wrong
       screen: StreamListCard,
       navigationOptions: {
         tabBarLabel: (props: TabNavigationOptionsPropsType) => (