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.
AppIds are used to lookup the actual contract addresses of each app.
Because of different registry names (open.aragonpm.eth vs. aragonpm.eth)
we have to use different ids in the local dev chain and in the
testnet/mainnet. To allow this we need to set the appids dynamically.
There is an open aragon issue to solve this and also allow to use
open.aragonpm.eth in the devchain by default.
https://github.com/aragon/aragen/issues/10
