Use format instead of type property for Content deserialization

While type works fine for deserialization from CouchDB, it is not part
of the public v3 API. The format property is available in both cases.

src/main/java/de/thm/arsnova/entities/Content.java

@@ -12,7 +12,8 @@ import java.util.Objects;
 @JsonTypeInfo(
 		use = JsonTypeInfo.Id.MINIMAL_CLASS,
 		include = JsonTypeInfo.As.PROPERTY,
-		property = "type"
+		property = "format",
+		visible = true
 )
 public class Content extends Entity {
 	public enum Format {

src/main/java/de/thm/arsnova/entities/serialization/CouchDbDocumentModule.java

@@ -36,8 +36,12 @@ public class CouchDbDocumentModule extends SimpleModule {
 		context.setMixInAnnotations(Entity.class, CouchDbDocumentMixIn.class);
 		context.setMixInAnnotations(de.thm.arsnova.entities.migration.v2.Entity.class, CouchDbDocumentV2MixIn.class);
 		context.registerSubtypes(
-				new NamedType(Content.class, Content.class.getSimpleName()),
-				new NamedType(ChoiceQuestionContent.class, ChoiceQuestionContent.class.getSimpleName()),
+				new NamedType(Content.class, Content.Format.TEXT.toString()),
+				new NamedType(Content.class, Content.Format.NUMBER.toString()),
+				new NamedType(ChoiceQuestionContent.class, Content.Format.CHOICE.toString()),
+				new NamedType(ChoiceQuestionContent.class, Content.Format.BINARY.toString()),
+				new NamedType(ChoiceQuestionContent.class, Content.Format.SCALE.toString()),
+				new NamedType(ChoiceQuestionContent.class, Content.Format.GRID.toString()),
 				new NamedType(Answer.class, Answer.class.getSimpleName()),
 				new NamedType(ChoiceAnswer.class, ChoiceAnswer.class.getSimpleName()),
 				new NamedType(TextAnswer.class, TextAnswer.class.getSimpleName()));