Fix: changing languages on the enrollment success page

[lalver1]

Closes #2058

On the enrollment success page, the Django set_language redirect view was redirecting the user to the enrollment index page when the language was changed. Looking at the Django documentation, adding a next parameter in the POST data and explicitly including the enrollment success page url in the context seems to be what was missing.

For all other views, when changing languages, I think we don't see this behavior because the Referer request header contains the address of the same view, but for the enrollment success view, Referer is /enrollment/ (I think because it is called from /enrollment/ initially), so using the next parameter is required.

[github-actions[bot]]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
benefits/enrollment
views.py
Project Total

This report was generated by python-coverage-comment-action