Do not allow changing the timeline template if the status is not New

[ThomasKranitsas]

Temporarily, disable the ability to change the timeline template on a challenge if the status is not New.

Make sure to add a // TODO comment with a Tech Debt not so we can track and revert this change once we phase out the legacy system which is currently causing issues with that feature.

Also, update the ChallengeType model to include a defaultTimelineTemplate that will be used if a template ID is not provided in the request body.

cc @rootelement