Use moment-timezone and zone id to get user's date

(cherry picked from commit cc725aa66ce011f17aa8b0819ae5f3da180e3908)
Use moment-helper to calculate user's local date

(cherry picked from commit 7e28b96cc833766c1548eba3e752ccdfb09c2fdb)
Timezone offset should be a number

(cherry picked from commit 3ea8542ed65d41581643922791c867285b0f54bd)
Remove unnecessary parameter from Graylog2Time

Now we rely on moment-helper to get the right tz offset.

(cherry picked from commit 4824d911cbc78e0e33a597715cecee1746bd852c)