Allow for modification of registration, submission, and checkpoint submission start times

[jmgasper]

Topcoder would like the ability to stagger these phases:

• Registration
• Submission
• Checkpoint submission

Currently, the v5 challenge API only has a "start date", which just sets all 3 to the same start time. We need the ability to optionally set these 3 phase start date / times individually.

The default should be all starting at the same time, like it does now, but the API should allow for staggering them. These new fields should be additional to what's in the API now, we don't want to change the existing API because that would involve changing multiple platform tools.

Update Swagger and Postman

Please ensure you update Swagger and Postman to show usage of the API updates.

Update tests

Please ensure the tests are updated to show usage and test the new fields