Voting: add role for FAST_EXECUTE

[sohkai]

Note: this has been closed to be tracked in the "wishlist for future upgrades".

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Fast execution has come under some stress since we originally designed the Voting app.

aragon/aragon-apps#886 (in next) allowed the app to configure whether early executions should be enabled.

Now there are users who would like to enable early execution, but protect it behind a role so that only privileged accounts can expedite a decision (e.g. a security patch). Ideally, they would like to do so if the min. quorum has been reached (but not necessarily un-challengeable support over the entire voting power!).

An example to illustrate the above:

• 60% quorum, 100% support
• A protected account can execute votes early once a vote gets to 60% quorum and is currently meeting 100% support

[sohkai]

Closing for now; we will consider this for future contract upgrades.