Fix for #829: Under maintenance project should not get back to "Draft…

[niksj]

Fixes 829: Under maintenance project should not get back to "Draft" after maintenance ended

[osarrat]

This test doesn't seem to match with issue description. Your line of code sends an error if a project model status which is Used and NOT under maintenance is changed to Draft. But actually, the issue appears for project model in Used and under maintenance status.

[osarrat]

Hi !
The fix you proposed seems to be partial and not completely correct. I've reported the error of understanding directly in a line of code comment.
The fix is also partial because the issue occurs when a user unchecks the "Under maintenance" checkbox and clicks on "Save", then the project model is sometimes shift automatically to "Draft" status. Your fix doesn't seem yet to deal with this case.
A bit more work is still required to finish it. :)