Spin out Explorer as an open source project

Following on from ACE-2091, we need to "spin out" the Explorer UI codebase as an open source project. The guidelines for doing this are documented here.

The spun out project should be available as an AMP that can be deployed to, and functions on the latest released versions of Alfresco on-premises (i.e. Community & Enterprise). At that point maintenance is handed over to the community and further updates (i.e. to keep it functional on newer versions of Alfresco) are the responsibility of the community-led project team that forms around the project.