Reader role give user access to system related information via System menu

When a user is allocated to the Reader role, system information is available to the user via the System menu.

## Expected Behavior

When assigning the Reader role to a user, the expectation is that the user will not have access to the System menu and system information.

## Current Behavior

When assigning a user to the Reader role, the user has access to the System menu and info about the cluster and its configuration.

## Possible Solution

A potential solution to the problem is to have a role to define a user that only needs to see messages in their allocated streams, with no access to system related information.

## Steps to Reproduce (for bugs)

1. Create a user
2. Assign the user to the Reader role
3. Log in as the new user
4. The user has access to the System menu and system related information

## Context

We are hosting a Graylog server which is used by multiple components as a centralized logging solution. An external company is hosting a system inside of our cluster, and are also writing messages to Graylog. We want to ensure the external company only has access to messages written by their application and no access to our own messages.

To achieve this we have set up a stream filtering based on the application parameter, and it works brilliantly.

However, when creating a new user for the external company and assigning the Reader role to the user, the external company's user has access to the System menu which is showing information about the installation we would rather not make publicly available.

In a perfect world, the external user will only have access to the Streams, Alerts, and Dashboards menu and not the Systems menu at all

## Your Environment

* Graylog Version: 2.4.0+2115a42
* Elasticsearch Version: Version: 5.6.5, Build: 6a37571/2017-12-04T07:50:10.466Z, JVM: 1.8.0_162
* MongoDB Version: db version v2.6.10
* Operating System: Ubuntu 16.04.3 LTS
* Browser version: Chrome