LP#1944765: enable the Apache module mod_headers

LP#1944765: enable the Apache module mod_headers

This patch enables mod_headers for Debian Stretch and Buster
and Ubuntu Bionic and Focal during installation. Without
mod_headers, the Angular staff client application can have
its index page cached for 24 hours on a stock system, meaning
that browsers will not consistently fetch the updated app
after an upgrade.

This patch does not change the Fedora Makefile as mod_headers
is enabled by default on RHEL-like distributions.

UPGRADE NOTES
-------------
Existing Evergreen systems on Debian and Ubuntu should
do the following to ensure that mod_headers is active:

sudo a2enmod headers
sudo systemctl restart apache2

To test
-------
[1] Perform a fresh installation of Evergreen on Stretch, Buster,
    Bionic, or Focal.
[2] Verify that mod_headers is enabled.
[3] (Optional) Fetch the Angular staff client index page, e.g.,
    /eg2/staff, and verify that the response headers call for
    not caching it.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>