Make any node field be an analysis variable, then use form to set on create

Split out from #6 - Analysis templates polish

Be able to make anything an analysis variable, and then at launch time, a form comes up where you can select the fields and go

> In create_analysis_from_template if not template_run.is_populated() - then redirect to a page where you can fill in remaining fields. Take them from node editor forms? Then once you've set all variables, fill them in and execute the analysis template

We know the node class, and can work out the form for that, I think it's just a matter of going into the form and altering what you can see, what are required etc.

However I'm not sure about the template rendering side, there are a few ways that we do that - might be tricky