OpenGov: Make referenda free to participate in

[gavofyork]

Unvoting/undelegating should be free in a rate limited way

Introduce a new dispatchable force_unlock, a new config item FreeForceUnlockRate: BlockNumber and a new storage item LastFreeForceUnlock: map AccountId -> BlockNumber. force_unlock should go through all voting over all classes, remove votes/delegations with zero conviction then unlock as much of the account's funds as possible. A fee is not charged for the transaction if this happens more than FreeForceUnlockRate blocks after the current value of LastFreeForceUnlock - in this case LastFreeForceUnlock is updated to now.

Voting which does not replace any previous vote should be free

Introduce a new storage item FreeVotes: map AccountId -> (u32, BlockNumber) and a new config item FreeVoteAmountAndDecay: (u32, BlockNumber). When voting, if now - FreeVoteAmountAndDecay.1 >= FreeVotes.1 then FreeVotes.0 is halved and FreeVotes.1 becomes now. Voting is free as long as FreeVotes.0 < FreeVoteAmountAndDecay.0 and there was no previous vote for the given proposal.

Delegating which does not replace any previous delegation should be free

Introduce a new variant into Voting: Consent(Delegation<...>). This is essentially the same as the Delegating variant except that no upstream votes have been altered. Introduce a new dispatchable consent, similar to delegate in all respects except that it uses the variant Consent instead of Delegating and does not alter any vote counts. It is always free to call as long as the account/class is not already voting, delegating or consenting.

Introduce a new dispatchable activate_consent callable by the target of a Consent vote. This is paid as standard, and results in the vote becoming a standard Delegating vote as well as all associated bookkeeping happening.

Once done wallets should be pushed to integrate a standard

With these changes, voting and delegating should now be free to perform. Wallets should retain voting/delegating preferences and enact them as soon as it is both free to do so and free to call the corresponding force_unlock immediately afterwards. Though the latter may not be needed, it's possible it would be should the user decide to transfer funds shortly afterwards.

Fund transferal should use force_unlock unless there are already enough unlocked funds in the account. This should be batched with the transfer in order to make the operation similarly trivial to a normal transfer.

A special option should always be given for abstention.