added missing decorator for classes that use Angular features

src/app/submission/sections/models/section.model.ts

@@ -1,4 +1,4 @@
-import { Inject, OnDestroy, OnInit, Directive } from '@angular/core';
+import { Component, Inject, OnDestroy, OnInit } from '@angular/core';
 
 import { Observable, Subscription } from 'rxjs';
 import { filter, startWith } from 'rxjs/operators';
@@ -14,7 +14,10 @@ export interface SectionDataModel {
 /**
  * An abstract model class for a submission edit form section.
  */
-@Directive()
+@Component({
+  selector: 'ds-section-model',
+  template: ''
+})
 export abstract class SectionModelComponent implements OnDestroy, OnInit, SectionDataModel {
   protected abstract sectionService: SectionsService;