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/comment-list/comment-list.component.html

 <div fxLayout="row" fxLayoutAlign="center">
   <div fxLayout="column" fxLayoutGap="20px">
+    <mat-progress-spinner *ngIf="isLoading" mode="indeterminate"></mat-progress-spinner>
       <div *ngFor="let comment of comments">
         <mat-card>
           <mat-card-content>