LP1828468 Keyboard navigation for main navbar

LP1828468 Keyboard navigation for main navbar

In the main navbar, all items except the home icon are unreachable using
only a keyboard. This includes the logout/change operator menu on the
far right.

This is caused by the lack of an href attribute on the <a> tags, which
prevents the link from having a focus state.

This patch adds tabindex="0" to the parent menu items and the submenu
items that use (click) or routerLink. Submenu items are also given the
ngbDropdownItem directive, which allows up/down arrow navigation within
submenus.

The patch does not include CSS to make the focus state prettier; a
separate patch for style is forthcoming.

Related: Bug #1814978

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