Refactor all DTO conversions

DTO conversions will now be done on the DTO itself again instead of in a Service. For this, a new component is added, replacing the DTOConverterService.

This component is called SpringContext and can be used to statically get the current thread's spring ApplicationContext.

src/main/java/nl/tudelft/librador/dto/create/Create.java

@@ -32,19 +32,22 @@ public abstract class Create<D> extends Validated {
 	 *
 	 * @return The created entity.
 	 */
-	public D apply(ModelMapper mapper) {
+	public D apply() {
+		validateOrThrow();
+
+		ModelMapper mapper = createMapper();
+		configureMapper(mapper);
+
 		D data = mapper.map(this, clazz());
 		postApply(data);
 		return data;
 	}
 
 	/**
-	 * Configures the {@link ModelMapper} that will be used by the Create application.
-	 *
-	 * @return The {@link ModelMapper} created for mapping the Create DTO.
+	 * Configures the {@link ModelMapper} that will be used by the Create DTO.
 	 */
-	protected ModelMapper createMapper() {
-		return new ModelMapper();
+	protected void configureMapper(ModelMapper mapper) {
+		// Leave empty
 	}
 
 	/**