Customvar view helper: show structured data

refs #7569
Customvars: use new helper

refs #7569
monitoring/css: style structured custom vars

refs #7569
Dataview\Customvar: add new is_json column

refs #7569
Ido\CustomvarQuery: add is_json

This has to be done in a backward compatible way, we check for IDO
>= 1.12.0.

refs #7569
MonitoredObject: decode structured customvars

fixes #7569