Server maintenance message via Apache config

This adds a custom maintenance message which you can issue in your
Apache config by setting the environment variable "MAINTENANCE_MESSAGE"
and restarting/reloading Apache.  This message appears in the TPAC for
both staff and patrons in the form of a red banner across the top of the
screen.

Signed-off-by: James Fournie <jfournie@sitka.bclibraries.ca>
Signed-off-by: Dan Scott <dscott@laurentian.ca>