Fix model lineage and create Python model UI form

[ajayjayendran]

Model Lineage

• Fix lineage for Python models. Python models now show their real upstream
  sources in the lineage graph, without the extra placeholder node that used to appear.

Create Python Model

• Group options match your project. The Create Python Model form now lists the
  groups from the project you've selected, and refreshes when you switch projects.
• Fix: creating a Python model Creating a Python model and loading
  its DAG and group options now work as expected.