Voting: Make early execution configurable

[facuspagnuolo]

Related to aragon/client#590

This PR aims to turn the fact that votes can be early executed once they have already been decided, into a configurable option.

[coveralls]

Coverage increased (+0.04%) to 97.105% when pulling 757cfed on early_execution_config into 98c30e9 on voting_delegation_app.

[bingen]

Looking good.
I guess changing the parameter could be confusing for users, specially for ongoing votes, so maybe we should ask for the preference during onboarding and maybe burn this new permission by default.
The way I see it, if an org wants to combine votes with early execution and without, they should have 2 separate instances, the same way we do now for different quorums.
I even wonder if it should be better to set this at init time only and remove the setter and the permission.

[luisivan]

Included this for 0.9, the next release that will introduce smart contract changes. Thanks @facuspagnuolo!

[sohkai]

Similar to the comment on the overrule window, what do you think about storing this as part of each vote?

It seems a bit weird that an existing vote can have its early execution behaviour change from underneath it. It causes a rather awkward (and error-prone) UX where you'd want to wait until any sensitive open votes are closed before executing a change to this parameter, rather than doing it whenever you wanted.

It starts to get a bit costly, but both the overrule window and the early execution variables can take up one slot in the Vote struct since they're being introduced together.

[facuspagnuolo]

@sohkai thanks for the review! Already addressed your comments, please LMKYT!