LP1830432: Make the org-family-select reusable

LP1830432: Make the org-family-select reusable

This commit removes Bill Erickson's automagic org unit select with
+Ancestors and +Descendants checkboxes from the admin-page component,
and gives it a component of its own, called <eg-org-family-select>.

This commit also makes it compatible with [(ngModel)], reactive forms,
and any custom Angular validators you might want to throw at it.
Examples of all three are available in the sandbox.

Also includes some component tests.

Signed-off-by: Jane Sandberg <sandbej@linnbenton.edu>