contribution refunds

[4meta5]

introducing non-refundable contributions fixed #84 but now we have this other problem

bounty4 proposal

1. A Permill threshold is set in the field of the struct BountyOf<T> to represent the required vote threshold to veto decisions made by the original funder. Votes can be triggered by any contributor (tracked in double_map BountyId, AccountId => Option<BalanceOf<T>>).

Another threshold could be set for the required threshold of support from contributors to trigger the second vote on a decision. This adds more friction for vetoing spending decisions.

2. Another threshold to trigger refunds in proportion to historic contributions. This drain the account associated with the Bounty to return remaining funds in proportion to contributions. It would NOT reverse already executed spends.

(next)

Could add more thresholds to enable the group to propose and pass spends which are not veto-able by the representative/initial funder. This would have significant overlap with bank.

Threshold for selecting a new representative (waiting on borda count module)