Display Header Menu only when you have access to the section

## Expected Behavior
When you are not admin user you might have header menu options that are empty or you can't do any useful. Those menu points should be only visible when you will have something behind to work/look at.

## Current Behavior
Most prominent is the "Enterprise" Menu option visible and clickable for non admin users when the enterprise plugins are not installed.

## Possible Solution
Display the menu options only when you are able to use/work with them.

## Context
It is not the best user experience if you are a normal user having the option "enterprise" available but nothing behind that is useful/possible to use for you.

When a user with the admin roles has this available that person could get a free enterprise license or similar, but the normal user does not have any options.

Just be a little more annoying to normal users would be fine.

## Your Environment

* Graylog Version: 3