Combine the "Upload file" with the metadata form during ingest

[rosiel]

This is in a way a throwback to 6, but some users would like a single form instead of a stepped form (as it relates to the metadata vs file upload).

In the backend, the way those forms are handled are very different. It is possible, but would require some intermediary handling.

It's not much of an advantage unless we can also get rid of the "select a content model" and "select a form" steps, addressed in #7