Copilot payments: Remove Challenge field on Add a payment

[jmgasper]

The copilot payment tracking should keep track of the challenge ID and apply it automatically. The user shouldn't be expected to do this.

No open challenge

If the user selects a project from the drop down and there is not currently a copilot payment challenge created, the topcoder-x processor should be expected to created one.

Existing challenge

If a copilot payment challenge already exists for a project from the drop down, then the existing challenge is just updated when a new payment is added, edited, or removed.

In either case, the Topcoder-X system is handling the challenge ID. The copilot should NEVER have to know the ID of a challenge.