nav panel: refactor to use vuex mapState

src/components/nav_panel/nav_panel.js

@@ -1,4 +1,5 @@
 import followRequestFetcher from '../../services/follow_request_fetcher/follow_request_fetcher.service'
+import { mapState } from 'vuex'
 
 const NavPanel = {
   created () {
@@ -9,17 +10,11 @@ const NavPanel = {
       followRequestFetcher.startFetching({ store, credentials })
     }
   },
-  computed: {
-    currentUser () {
-      return this.$store.state.users.currentUser
-    },
-    chat () {
-      return this.$store.state.chat.channel
-    },
-    followRequestCount () {
-      return this.$store.state.api.followRequests.length
-    }
-  }
+  computed: mapState({
+    currentUser: state => state.users.currentUser,
+    chat: state => state.chat.channel,
+    followRequestCount: state => state.api.followRequests.length
+  })
 }
 
 export default NavPanel