forms: store the formdata under source_data instead when creating the workflow.

[david-caro]

In order to be able to restart a wokrflow reusing the same data that was used to start it the first time, we have to unify between submissions and harvests where to put that data, and that key will be from now on source_data under the workflow extra_data.

[david-caro]

This is a bit more complex than I though, the source_data must have information about both, the extra_data that it would set in the workflow + the data, so it must have the structure:

"source_data": {
    "extra_data": {
        ...
    },
    "data": {
        ...
    }
}

[david-caro]

It also requires adding a step/call in the restart to restore it.