Fix: avoid fetching file status after navigating

Without invalidating the timer, navigating from one survey to another
would keep the status for both surveys' files to be fetched, yielding
bugs in the UI.

See #2350
See #2362

assets/js/components/respondents/RespondentIndex.jsx

@@ -102,6 +102,14 @@ class RespondentIndex extends Component<Props, State> {
     }
   }
 
+  componentWillUnmount() {
+    const timer = this.state.filesFetchTimer
+    if (timer) {
+      clearInterval(timer)
+      this.setState({filesFetchTimer: null})
+    }
+  }
+
   fetchRespondents(pageNumber = 1, overrideFilter = null) {
     const { projectId, surveyId, pageSize, filter, sortBy, sortAsc } = this.props
     const _filter = overrideFilter == null ? filter : overrideFilter