buildsys: install templates for test notifications

buildsys: install templates for test notifications

The notification stack loads handlebar templates for notifications
from /usr/share/proxmox-backup-server/templates/default/. This commit
modifies the build system to install template files from the
'templates' directory at that location. First, we only have templates
for test notifications.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
Tested-by: Gabriel Goller <g.goller@proxmox.com>
Reviewed-by: Gabriel Goller <g.goller@proxmox.com>
Tested-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>