Current and Longest streak calculation fixed

Minor refactoring and unit tests added

After CR: user-stats file moved to util folder, export keywork added to exported functions, new line added at the end of gulp file

User-stats-test file moved to replicate user-stats path in test folder

server/utils/date-utils.js

@@ -0,0 +1,11 @@
+import moment from 'moment';
+
+// day count between two epochs (inclusive)
+export function dayCount([head, tail]) {
+  return Math.ceil(
+    moment(moment(head).endOf('day')).diff(
+      moment(tail).startOf('day'), 
+      'days', 
+      true)
+    );
+}