[16.0] base_maintenance: style on ButtonBox and colspan

[Christian-RB]

Hi,

Currently working on maintenance_timesheet migration and once I inherit the button_box from base_maintenance the following error is thrown:

Also the timesheet colspan seems wrong set (is taking more room than needed):

The PR to fix it: #351
And if you want to check the actual error it can be done on this PR runbot: #352

Regards