fix(stark-demo): add baseHref dynamically via Angular provider to fix Showcase when published in GitHub pages

[christophercr]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #466

What is the new behavior?

The base href of the showcase is set dynamically via the Angular APP_BASE_HREF provider as an environment provider instead of setting it in the angular.json file in order to handle the final location of the app in GitHub pages (defined in environment.prod.ts).

The handling of the environment files in Webpack is works correctly now due to some minor fixes in the build-utils.js file.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information