message: fix: disable install button when url input is empty
  fixes
diff: placeholder="{{ 'URL-INSTALLER:PASTE-URL' | translate }}"
  [ngModel]="sourceUrl"
  (ngModelChange)="onSourceUrlChange($event)"
  + required
  + #urlInput="ngModel"
  />
  <div class="flogo-installer-url__button-container">
  <button class="flogo-button--secondary" (click)="onCancelAction()">
  {{ 'URL-INSTALLER:CANCEL' | translate }}
  </button>
  - <button class="flogo-button--default" (click)="onInstallAction()">
  + <button
  + class="flogo-button--default"
  + [disabled]="urlInput.invalid"
  + (click)="onInstallAction()"
  + >
  {{ 'MODAL:INSTALL' | translate }}
  </button>
  </div>
  