Error when loading `assets/mode-pipeline.js` for pipeline source code editor.

## Description

When opening the source code editor for a pipeline rule, the following browser console error occurs in production:
```
Refused to execute script from '/assets/mode-pipeline.js' because its MIME type ('text/html') is not executable, and strict MIME type checking is enabled.
```
In the dev environment the console error is:
```
Uncaught SyntaxError: Unexpected token '<' (at mode-pipeline.js:1:1)
```

In both cases the JS file contains HTML. I just checked it for development, but the related JS code is still being executed.

## Your Environment

* Graylog Version: dev version for `5.2`, error occurs also with `5.1` and `5.0`