Incorrect Timezone Data for GMT+8

Setting a User Timezone to GMT+8 results in a functional timezone of GMT-8

## Expected Behavior
(Output Example when set to Australia/Perth)

User adamn:
2018-02-27 16:19:07 +08:00
Your web browser:
2018-02-27 16:19:07 +08:00
Graylog server:
2018-02-27 08:19:07 +00:00

## Current Behavior
When user 'adamn' Timezone is set to GMT+8

User adamn:
2018-02-27 00:19:07 -08:00
Your web browser:
2018-02-27 16:19:07 +08:00
Graylog server:
2018-02-27 08:19:07 +00:00

## Steps to Reproduce (for bugs)
1. Create or Select a user (not main Administrator account)
2. Set timezone to GMT+8 and Apply
3. Navigate to Overview page
4. Observe time debug data
5. Reset user to a +8 timezone, eg Australia/Perth.
6. Navigate to Overview Page
7. Observe correct timezone data.

## Context
Confused the bejeezus out of me when trying to correlate some log messages to the source system!

## Your Environment
Basic install following Docker instructions from Documentation.

* Graylog Version: 2.4.0-1 (default version for Docker installs)
* Elasticsearch Version: 5.6.2
* MongoDB Version: 3
* Operating System: Debian Jessie
* Browser version: Chrome Version 63.0.3239.132 (Official Build) (64-bit)