Adding inputs not possible when Graylog URL includes path

## Description

Having this plugin installed and serving Graylog from URL that includes additional path segment (e.g. /graylog/) breaks adding new inputs. Without plugin installed Graylog functions correctly.

## Steps To Reproduce

1. Run Graylog with URL that has path segment (e.g. http://localhost:9000/graylog/).
1. Try to Launch any input -> configuration form doesn't open.
1. Additional Launch button clicks cause Javascript errors in console.

## Environment

- Graylog Version: 3.2.6 (also tested with 3.3.7)
- Elasticsearch Version: 5.6.9
- MongoDB Version: 4.0
- Browser Version: Chromium 85.0.4183.121