Facilitate production testing of analysis code

[bloodearnest]

Right now, to test some changes to interactive analysis code on the server, we need to:

a) commit/push those changes interactive-templates
b) bump interactive-templates in job-server, and deploy it
c) walk through the form to submit a job, which will trigger the usual emails/output-checking flows.

This is not ideal, as it means

1. our in development changes will be exposed to users before they've actually been tested on the server
2. laborious for developer (especially if form changes are required on the django side)
3. laborious for output-checkers

To test that new bits of analysis code development (e.g. moving to databuilder, testing some performance options, making a new analysis), it would be better if there was a way to render/commit/push and then submit a job without going through the full interactive pipeline. Basically, a special purpose workspace/repo, which is soley for testing execution of in-development analysis code.

This means we
a) wouldn't expose development code to the user or touch job-server at all
b) wouldn't need to do the form plumbing to test code
c) wouldn't need to bother output-checkers, as we wouldn't be releasing anything.

We'd only need to do all the above once the code is ready to deploy to users.

It would mean that interactive-code developers can write and test analysis code just like it was a regular study, basically, and ignore job-server all together.

This is not on the critical path to real users using OSIv2, but it would speed up development, and once we have users using it for real, then would become essential.