LP#1831783: improvements to the Angular org-select widget

LP#1831783: improvements to the Angular org-select widget

- add reset() method
- add method for retrieve current selected OU. This can be invoked
  from local template references in parent templates like this:

    <button ... (click)="doAction(ousel.selectedOrg())" ...
    ...
    <eg-org-select (onChange)="doAction($event)" #ousel></eg-org-select>

Sponsored-by: MassLNC
Sponsored-by: Georgia Public Library Service
Sponsored-by: Indiana State Library
Sponsored-by: CW MARS
Sponsored-by: King County Library System
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>