Factor experiment create logic to both API and UI

[miketynes]

The if ladder in experiment.py here:

ESCALATE/escalate/core/views/experiment.py

Line 261 in d270aa6

# begin: template-specific logic

Needs to also exist in this REST API method:

ESCALATE/escalate/rest_api/viewsets.py

Line 161 in d270aa6

def create(self, request, *args, **kwargs):

This will enable exp-specific logic in the REST API, as it currently only exists in the UI.