Restructure the Imports and Exports

[KevinStarWars]

Angular’s best practices (https://angular.io/guide/styleguide) show that transitive imports should be used through a CoreModule. The CoreModule is supposed to be imported once into the AppModule. A CoreModule makes the import and export statements much more readable and also leads to a higher maintainability of the code.
All material modules should be placed inside a dedicated MaterialModule, which is then imported into the CoreModule.
Unused export statements should also be removed, since they do not provide any value.
Tasks, which derive from this issue:

• create a CoreModule inside a core folder below the src folder.
• restructure the imports so that multiple imports are avoided and the imports are clear.
• restructure the exports so that unused exports do not exist.