showcase: ExampleViewer does not take into account the baseUrl while fetching the example files

[christophercr]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[X] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

The ExampleViewer does not show any example files due to an Http error while fetching the files:

GET https://stark.nbb.be/assets/examples/keyboard-directives/on-enter-key-directive.html 404 ()

The problem occurs because the Http request triggered by the FileService to fetch the files does not take into account the baseUrl of the app when this is deployed on GitHub Pages.

Expected behavior

The FileService should fetch the files targetting the correct Http, for example in the latest Showcase:

GET https://stark.nbb.be/showcase/latest/assets/examples/keyboard-directives/on-enter-key-directive.html 

Minimal reproduction of the problem with instructions

Go on Stark showcase and go to any demo page.
In the demo page click on the "View source" button of the ExampleViewer component.
The example files will not be displayed and an error will be shown in the console.

Environment

Angular version: X.Y.Z
Stark version: 10.0.0-alpha.4