LP2019032 Keyboard support for PO line items

LP2019032 Keyboard support for PO line items

Reworks links, buttons, and form labels in the single PO view.

* <a click()> is now <button type="button">
* Items in dropdown lists have ngbDropdownItem directive for
keyboard up/down arrow support
* Icons are now aria-hidden (except "close," which is fine as-is)
and their parent elements have ARIA labels where necessary
* Text inputs have labels instead of placeholders

Signed-off-by: Stephanie Leary <stephanie.leary@equinoxoli.org>
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>