Handle when a bounty gets paid out and then the issue reopened

Right now if a bounty gets paid out and the issue subsequently gets re-opened, the issue on Bountysource retains the "paid out" tag and previous Backers list. This is confusing.
- Remove the Paid Out tag
- TBD figure out what to do with previous Backer / Developer info... move to a history/activity tab?

Example:
https://www.bountysource.com/issues/538219-issue-view-doesn-t-handle-github-markdown-checkboxes-properly
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