Request Enketo IDs during request when form is created or published

[matthew-white]

This PR addresses three cases related to getodk/central#385:

• When a form is created for the first time, request an enketoId for the draft.
• When a form is published for the first time, request an enketoId and enketoOnceId for the form.
• When a form is created for the first time and immediately published (via the ?publish=true flag of the API, thereby skipping a draft), request an enketoId and enketoOnceId for the form.

These are the last cases to address, so this PR closes getodk/central#385.

Does this change require updates to the API documentation? If so, please update docs/api.md as part of this PR.

I quickly searched the API docs for "Enketo", but I didn't see anything that needed to case.

Before submitting this PR, please make sure you have:

• run make test-full and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced

[matthew-white]

Notes from interactive code review.

I may make additional changes before merging, especially adding more tests. @ktuite and I will review significant changes next week.