LP#1831907: bump up ng-bootstrap

LP#1831907: bump up ng-bootstrap

This bumps up ng-bootstrap to ^4.2.0, which as of the date
of this patch resolves to 4.2.1. Also as of the date of this
patch, ng-bootstrap 5.1.0 is available, but cannot be used
prior to upgrading Angular from 7 to 8.

To test
-------
[1] Install the patch.
[2] In the sandbox (/eg2/en-US/staff/sandbox) verify that keyboard
    navigation for the grid action menu's dropdowns works.
[3] Look for other oddities in the Angular client.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>
Signed-off-by: Bill Erickson <berickxx@gmail.com>