Add text-content data model and implemented inheritance for choice-content

Add constructors for all contents

src/app/answer-option.ts

 export class AnswerOption {
+  constructor(label: string, points: string) {
+   this.label = label;
+   this.points = points;
+  }
   label: string;
   points: string;
 }

src/app/choice-content.ts

 import { AnswerOption } from './answer-option';
+import { Content, Format } from './content';
 
-export class ChoiceContent {
+export class ChoiceContent extends Content {
+  constructor(roomId: string, subject: string, body: string, options: AnswerOption[], correctOptionIndexes: number[], multiple: boolean) {
+    super();
+    this.revision = '1';
+    this.roomId = roomId;
+    this.subject = subject;
+    this.body = body;
+    this.round = 1;
+    this.format = Format.CHOICE;
+    this.options = options;
+    this.correctOptionIndexes = correctOptionIndexes;
+    this.multiple = multiple;
+  }
   options: AnswerOption[];
-  correctOtionIndexes: number[];
+  correctOptionIndexes: number[];
   multiple: boolean;
 }

src/app/text-content.ts

+import { Content, Format } from './content';
+
+export class TextContent extends Content {
+
+  constructor(roomId: string, subject: string, body: string) {
+    super();
+    this.revision = '1';
+    this.roomId = roomId;
+    this.subject = subject;
+    this.body = body;
+    this.round = 1;
+    this.format = Format.TEXT;
+    this.formatAttributes.clear(); // API: formatAttributes = Map.empty();
+  }
+}