[Launchpad] When an issue is marked as a duplicate, mark as "Closed" on Bountysource [$50]

Example:

LP: https://bugs.launchpad.net/elementaryos/+bug/1247721
"This bug report is a duplicate of: Bug #1080244: No OwnCloud Calender Support [$40 or $130]."

Bountysource issue: https://www.bountysource.com/issues/1066044 --> should be Closed
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There is a **[$50 open bounty](https://www.bountysource.com/issues/1842977-launchpad-when-an-issue-is-marked-as-a-duplicate-mark-as-closed-on-bountysource?utm_campaign=plugin&utm_content=tracker%2F47&utm_medium=issues&utm_source=github)** on this issue. Add to the bounty at [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F47&utm_medium=issues&utm_source=github).