Bug 27940: Fix missing email in OpacMaintenance page

Bug 27940: Fix missing email in OpacMaintenance page

To reproduce:

KohaAdminEmailAddress = Some valid email address
OpacMaintenance = Show

OPAC/cgi-bin/koha/maintenance.pl shows this text:

"Koha online catalog is offline for system maintenance. We'll be back
soon! If you have any questions, please contact the site administrator."

"site administrator" is a link, but it only links to "mailto:", the
actual email address from KohaAdminEmailAddress is missing.

To test:
- Apply the patch
- Reload the maintenance page
- Click on the "site administrator" link and verify it contains the
  address from KohaAdminEmailAddress

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit df3a8d25482e10c748a52cb0a7e66ecf9b854364)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>