Add project selection to Topcoder X tab on Work Items

[jmgasper]

On the Topcoder X tab on Work Items, we need to add a drop down for the Topcoder project to launch the challenge under. This won't be editable after the challenge is launched.

The contents should come from the project API, using the user's token, so that the drop down only shows the projects the user has access to, similar to what we already do in the plugin setup.