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.

apps/contribution/arapp.json

@@ -9,6 +9,11 @@
       "name": "Manage token contract",
       "id": "MANAGE_TOKEN_CONTRACT_ROLE",
       "params": []
+    },
+    {
+      "name": "Veto contributions",
+      "id": "VETO_CONTRIBUTION_ROLE",
+      "params": []
     }
   ],
   "environments": {