refactor(stark-demo): cleanup of Stark Showcase application

[RobbyDeLaet]

Applied the same changes that where made to the Starter app.

ISSUES CLOSED: #422

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?

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

What is the current behavior?

The ShowCase application is not following the same guidelines and structure as the Starter application.

Issue Number: #422

What is the new behavior?

The ShowCase and Starter application are following the same guidelines and structure.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[coveralls]

Coverage decreased (-0.01%) to 94.246% when pulling c0a0163 on RobbyDeLaet:feature/CleanupShowcase into 0ced55c on NationalBankBelgium:master.