Navigation links broken if `http_external_uri` contains a path

[thll]

Expected Behavior

Navigation links should keep working when Graylog is hosted under a separate path.

Current Behavior

If the http_external_uri contains a path, or if the X-Graylog-Server-URL is set to a URL that contains a path, that path is duplicated in the Navigation links which renders them broken.

Possible Solution

Remove the duplication ;)

Steps to Reproduce (for bugs)

1. Set up a reverse proxy to serve Graylog at http://localhost/graylog/
2. Either configure Graylog with a http_external_uri = http://localhost/graylog/, or make sure that all requests reach Graylog with the X-Graylog-Server-URL set to http://localhost/graylog/
3. Notice e.g. the Search link in the navigation bar to point to the wrong URL http://localhost/graylog/graylog/search

Your Environment

• Graylog Version: 6.0.0-SNAPSHOT
• Java Version:
• OpenSearch Version:
• MongoDB Version:
• Operating System:
• Browser version: