Link to Enterprise is wrong when http_publish_uri doesn't point to /

## Expected Behavior

The link to the Enterprise page should work

## Current Behavior

The link to the Enterprise page is wrong

## Possible Solution

Don't use add an endpoint in the parameter http_publish_uri.

## Steps to Reproduce (for bugs)

1. Edit server.conf
2. Add an endpoint in the http_publish_uri. For example https://xxx:8443/graylog/
3. Restart the Graylog service
4. Try to open the Enterprise page. You will be redirected to https://xxx:8443/graylog/graylog/system/enterprise (note the graylog word repeated twice)

## Context

## Your Environment

* Graylog Version: 3.3.10
* Java Version: java-1.8.0-openjdk-headless-1.8.0.282
* Elasticsearch Version: 6.8
* MongoDB Version: 4.0
* Operating System: CentOS 7
* Browser version: Firefox 78 ESR