RFC: Slicer extension end-of-life procedure

[pieper (Steve Pieper (Isomics, Inc.))]

Very good point. We actually started discussing this the other day on the developer hangout and started working on a standardized scheme for the extension status.

2019.03.26 Hangout

Notes: Thanks to @Sam_Horvath, extension status will be standardized to Stable, Experimental and Unmaintained (or similar). The status will be used to dispatch the build results in appropriate groups: for SlicerPreview dashboard Stable|Experimental -> Extensions-Nightly Unmaintained -> Extensions-Nightly-Unmaintained code to update here in master branch for SlicerStable dashboard Stable|Experimental -> Extensions-X.Y-Nightly Unmaintained -> Extensions-X.Y-Nightly-Unmaintained co…