Add polling to queries

Subscriptions are the goal, since they are likely much more efficient
and scale, but for the purposes of a demo we think that just using built
in polling functionality of ApolloClient will do the trick.

Issue #44

src/client/components/Dashboard.js

@@ -56,6 +56,7 @@ class Dashboard extends React.Component {
               after: intervalScope.startTime,
               before: intervalScope.endTime,
             }}
+            pollInterval={2000}
           >
             {({ data, error, loading }) => {
               if (loading) {
@@ -101,6 +102,7 @@ class Dashboard extends React.Component {
           <Query
             query={RECENT_SENTENCES_QUERY}
             variables={{ after: moment().subtract(5, 'minutes').toISOString() }}
+            pollInterval={2000}
           >
             {({ data, error, loading }) => {
               if (loading) {