Allow web maps to be hidden from Explorer

Idea created by m0nki on Dec 17, 2018

    Similar to this idea (now implemented with the DEC 2018 release of AGO), also include a check box for Explorer. We have maps specifically for web apps and dashboards and maps for field use. It would be good to exclude these maps from showing up in Explorer. There is already enough noise with all the Living Atlas content that has no relevance to our work. Our field crews like things to be simple, and this would help by only displaying relevant content.