feat(nimbus): add results page button to export raw results_data json

[jaredlockhart]

Because

• The results page renders a curated view of results_data with no way to get the underlying payload out of the UI.
• Retrieving the raw analysis JSON currently requires Django admin or database access.

This commit

• Adds a ResultsExportView serving an experiment's results_data as a JSON file download.
• Adds an Export JSON button to the results page Overview header.

Fixes #17252

[jaredlockhart]

Local render of the new Export JSON button on the results page Overview header, against a dev stack with generated dummy data. The button now uses btn-primary, so it reads as a solid blue action matching the "Create Experiment" button in the page header rather than blending into the surrounding text.

In context on the full results page:

The endpoint was also exercised against the running stack:

$ curl -sI http://localhost:7001/nimbus/devolved-multi-tasking-projection/results/export/
HTTP/1.1 200 OK
Content-Type: application/json
Content-Disposition: attachment; filename="devolved-multi-tasking-projection-results.json"

[mikewilli]

Do you think it might be more useful to provide a query to get the data directly from BigQuery instead of downloading a (potentially large and confusing) JSON payload?

I guess a better question might be: who is this for?

[jaredlockhart]

@mikewilli I think we can do both! Here is just for the export, we can find ways to do the BQ side separately.