Resolve "Refine content-list-view"

Refactor content-list view

• Refactor whole view
• The content-list should get some enhacements
• Creators should be able to edit and delete single contents in the content-list view.

Several other fixes

• UI-/UX-Enhacements
• Refine content-choice-creation
• Adjust content-service
• Remove MardownComponent from app
• Remove FooterLoginDialogComponent from app

Closes #192 (closed)