[editor] Fix dynamic height of TextEditor dialog

Now the dialog scrolls and the buttons at the bottom can be reached even 
on low viewport height.

#294

projects/editor/src/app/services/dialog.service.ts

@@ -57,6 +57,7 @@ export class DialogService {
         defaultFontSize,
         clozeMode: false
       },
+      height: '600px',
       autoFocus: false
     });
     return dialogRef.afterClosed();

projects/editor/src/app/text-editor/rich-text-editor.component.css

+tiptap-editor {
+  height: 57%;
+}
 :host ::ng-deep div.ProseMirror {
-  min-height: 300px;
+  height: 99%;
   border: 1px solid;
 }