Merge FormsetStep and TableStep.

The FormsetStep is no longer needed, TableStep is used instead, with a
special version of DataTable, FormsetDataTable, that apart from all the
normal DataTable features also generates a Django Formset with all of
its data, and displays it the same way the FormsetStep did.

One issue is not addressed:

 * The user-provided data in the forms can be lost if that row gets
   updated by ajax autoupdate.

Change-Id: I5264b484f61212c21102739b803d40d9d31ae2cc
Merge "Merge FormsetStep and TableStep."