404 page when using inspector form after a normal update

[jonkri]

Describe the bug

@dracos wrote the following:

If a user has the Manage shortlist permission, then the referrer is stored when loading a report page, and that referrer is then used after the inspector form is submitted, to redirect back to. I think this is so we can try and send someone back to the page they came from - whether that's the /around page, their shortlist page, an all reports page, or so on, but I'm not sure it works in all situations.

In this case, my guess is the municipality made a normal update (which does a POST to /report/update). They then filled in the inspector form and submitted that, and then the auto-redirect after that presumably redirected back to /report/update (as a GET, as that was the referrer) which then returns the 404 as you say (as it's only used for POSTing update forms). I can't see any other way this could happen, anyway. I guess we could try and not use the referrer if the previous page was a POST, which could help with this precise situation, if it is that.

(https://groups.google.com/a/mysociety.org/g/fixmystreet/c/PoYaOZIEN0c/m/9W7DyQbBBgAJ)

To Reproduce

Steps to reproduce the behavior:

1. Log in as a user with the Manage shortlist permission
2. Visit a report
3. Make a normal update (not using the inspector form)
4. Click the link back to the report (the name of the report)
5. Make a report assignment using the inspector form

A 404 page is displayed.

Expected behavior

The report page should be displayed.

Screenshots

Desktop (please complete the following information):

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Smartphone (please complete the following information):

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Additional context

The assignment is carried out even though the 404 message is displayed.