Async Tasks

[dternyak]

With grant-project#116 in, we have the groundwork for async tasks, but we don't yet have specific tasks built out.

Here are a list of potential tasks we may want, but not necessarily all.

• Contribution failed emails. If we don’t confirm a contribution in some amount of time (say, 24 hours) we should send an email letting the user know it failed.
• Purge

[AMStrix]

As mentioned in #185 - arbiters should be unassigned from failed proposals, a task should perform this action after the proposal deadline has passed. Alternatively we could just disallow setting arbiters on proposals that have not achieved full-funding.

[dternyak]

Remove PENDING contributions, contributions that never get confirmed on chain, after 30 days without a relevant contribution.

[wbobeirne]

Closing this out in favor of a backlog issue for purging, #338