Feature: Make the CLI maintenance page content configurable

[abias]

The CLI maintenance mode (which is activated with admin/cli/maintenance.php --enable) outputs a rather ugly and unstyled

It would be nice if end users would see a friendly maintenance page like this one:

To achieve this, the admin simply has to place a climaintenance.template.html with the appropriate content into MOODLEDATA.
And, of course, Boost Union could then automate the file placement with a Boost Union setting