Add contribution veto function

Allows anybody with the VETO_CONTRIBUTION_ROLE to veto a contribution
and thus prevent that the contribution can be claimed.
So far like the ADD_CONTRIBUTION_ROLE any contributor is allowed to
veto.

arapp.json

@@ -5,6 +5,11 @@
       "id": "ADD_CONTRIBUTION_ROLE",
       "params": []
     },
+    {
+      "name": "Veto contributions",
+      "id": "VETO_CONTRIBUTION_ROLE",
+      "params": []
+    },
     {
       "name": "Manage contributors",
       "id": "MANAGE_CONTRIBUTORS_ROLE",