We can send multiple 'Completed' requests when we want to close a challenge

[LieutenantRoger]

Current:

On the challenge detail page, when we click the 'Confirm' button multiple times very quickly on the popup window after we 'Mark a challenge as Completed', the Work Manager continues send requests to the server.

Expected:

When a user clicked the 'Confirm' button for once, the button should not continue respond to the user's further click operations to avoid any requests sent to server.