Move reporting of validation status

Moving status indications of individual validations into the tree of
each validationPolicy is necessary, because the validation may have
different expiration times for the different policies that rely on it.

docs/design.md

@@ -426,26 +426,26 @@ validationPolicies:
   site-deploy-validation:
     url: https://deckhand/revisions/0/documents?schema=deckhand/ValidationPolicy/v1&name=site-deploy-validation
     status: failed
-    validationNames:
-      - deckhand-schema-validation
-      - drydock-site-validation
-      - promenade-site-validation
-      - armada-deployability-validation
-validations:
-  deckhand-schema-validation:
-    url: https://deckhand/revisions/0/validations/deckhand-schema-validation/0
-    status: success
-  drydock-site-validation:
-    status: missing
-  promenade-site-validation:
-    url: https://deckhand/revisions/0/validations/promenade-site-validation/0
-    status: expired
-  armada-deployability-validation:
-    url: https://deckhand/revisions/0/validations/armada-deployability-validation/0
-    status: failed
+    validations:
+      - name: deckhand-schema-validation
+        url: https://deckhand/revisions/0/validations/deckhand-schema-validation/0
+        status: success
+      - name: drydock-site-validation
+        status: missing
+      - name: promenade-site-validation
+        url: https://deckhand/revisions/0/validations/promenade-site-validation/0
+        status: expired
+      - name: armada-deployability-validation
+        url: https://deckhand/revisions/0/validations/armada-deployability-validation/0
+        status: failed
 ...
 ```
 
+Validation status is always for the most recent entry for a given validation.
+A status of `missing` indicates that no entries have been created. A status
+of `expired` indicates that the validation had succeeded, but the
+`expiresAfter` limit specified in the `ValidationPolicy` has been exceeded.
+
 ### POST `/revisions/{{revision_id}}/validations/{{name}}`
 
 Add the results of a validation for a particular revision.