Adjust tribute refunding condition

[dusan-maksimovic]

Closes #140

This PR does the following:

• Tributes can be claimed only if the proposal was among the top N and it received at least the specified percentage threshold of the total voting power.
• Tributes can be refunded only if the proposal wasn't among the top N or it was among the top N but it received less than the specified percentage threshold of the total voting power.
• Percentage threshold is specified through the instantiation message. For the deployed contracts, a proper migration is added that allows this parameter to be specifed.

[p-offtermatt]

I still think we should extract a method from refund_tribute and claim_tribute.
These two going out of sync is the kind of thing that will cause bugs in the future.

How about making it a function "TributeIsRefundable() -> (bool, str)", where the string is a message that explains the "why" and that can be used in error returns.
For example, if the proposal is in the top N and has the percentage, the return is:
false, "proposal is in the top N and has at least x% of the votes".
if its not in the top N:
true, "proposal is not in the top N"

if it does not have the percentage threshold:
true, "proposal does not have at least x% of the votes"

Then the message can be used to inform the error return. so for claiming, if "IsRefundable" returns true, it would go:
"Cannot claim tribute:" + message, so e.g. it would look like
"Cannot claim tribute: proposal is not in the top N"