Added all missing links to the-links-object.md.
Also removed Schema placeholder headings from event descriptions;
the intent is to add schemas to separate repository.

links.environment should have been links.artifact.

After renaming links.previous to links.previousVersions, it makes little
sense to keep previous activity executions in there. Actually identifying
the ActivityQueued of the latest previous execution may not be trivial,
however, so it should still be optional (besides, there's always a first
time).