Resolve "Adjust participant-room view"

Closes #135 (closed)

Proposed changes

• Adjust participant room view
• Added loading spinner to room-list, comment-list
• Adjust room-list join butoon

For testing

• Login as creator or participant and you should see the join buttons in room-list and the loading spinner before the list is loaded
• Login as participant, join a room & you should see a new room-view
• If you go to comments you should see the loading spinner

src/app/add-content/add-content.component.html

+<div fxLayout="column" fxLayoutAlign="start" fxLayoutGap="20px" fxFill>
+  <div fxLayout="row" fxLayoutAlign="center">
+    <mat-tab-group>
+      <mat-tab label="Text">
+        <div class="tab-container">
+          <app-creator-text-content></app-creator-text-content>
+        </div>
+      </mat-tab>
+      <mat-tab label="Choice">
+        <div class="tab-container">
+          <app-creator-choice-content></app-creator-choice-content>
+        </div>
+      </mat-tab>
+    </mat-tab-group>
+  </div>
+</div>