Handle situations where request has no User object, closes #1746

[alukach]

Proposed changes in this pull request

Going off of the description in this StackOverflow article, #1746 appears to be related to situations where we have a redirect from APPEND_SLASH=True. Typically, a request works its way through the middleware stack, is sent to a view, and then a response works its way through the middleware (in opposite order). If a redirect occurs from the CommonMiddleware middleware but before AuthenticationMiddleware, the AuthenticationMiddleware is skipped (meaning the user object is not added to the request) and the response is sent through all middleware. Our UserLanguageMiddleware expects that the request have a user property, but in the event of skipping the AuthenticationMiddleware this is not true.

We could do a re-org of the middleware, but it seems more robust to just handle situations where there is no user property on the request.

When should this PR be merged

ASAP, as this is a blocking bug.

Risks

None foreseen. I would have liked to write a test that worked through all the existing middleware, but felt that this was simpler and better for the sake of time.

Checklist (for reviewing)

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[amplifi]

Looks good. Works in dev VM.

[oliverroick]

🥇