Add a gradle task to make the getdown archive dir and JVL file

Each release of Jalview should have an archive DIR with JVL file to allow switching channel to that specific version.

This should be created at the same time as the release so that the exact same JARs can be used (with same build time in the properties), and therefore should be automated as a gradle task to be added to the "Getdown RELEASE Channel" build plan.

The archive is mostly a copy of the built release channel but with different getdown splash screen and an added JVL file with appbase pointing specifically to this version archive. The getdown.txt also needs to be adjusted to point to this version archive.