fix: resolve bulk upload Turbo error with PRG pattern

[mnietona]

Description

Following your feedback, I investigated the Turbo console error instead of just disabling Turbo.

The Problem:
When the bulk upload form is submitted successfully, the submit_bulk view directly returns a render() (HTTP 200 OK). Hotwire Turbo strictly enforces the Post/Redirect/Get (PRG) pattern and throws the Form responses must redirect to another location error if a POST request doesn't return a redirect.

The Implemented Solution (Current PR):
I modified the view.py to respect the PRG pattern.

1. On successful POST, it now returns an HttpResponseRedirect to the same URL, appending a ?success=1 parameter.
2. The view catches this GET parameter and renders the existing document_bulk.html success page.
   Result: The console is perfectly clean, and Turbo handles the navigation smoothly.

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Alternative Idea for the Future (Option B)

While this PR fixes the bug using the current UI, having a dedicated success page (document_bulk.html) and ?success=1 in the URL might feel a bit heavy.

As an alternative, we could completely remove this success page and use Django Flash Messages instead:

• After a successful POST, we redirect the user straight back to the course page (or the upload form).
• We display a global green success banner saying: "Merci ! Ton lien a bien été envoyé aux administrateurs pour traitement."

Let me know if you prefer to merge this PR as a quick fix, or if you'd like me to implement this "Flash Message" alternative instead!

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Manual Testing:

• Tested with a valid URL on local environment.
• Verified that the Turbo console error is completely gone.

Fixes #369

[C4ptainCrunch]

Please use the query parameter of reverse instead of crafting the url by hand: https://docs.djangoproject.com/en/6.0/ref/urlresolvers/#reverse

[C4ptainCrunch]

Also, could you try to have only ?success if possible ? Probably by treating it as a boolean

[C4ptainCrunch]

I prefer option A as users are often "blind" to banners and flash messages. If you could just address my 2 comments i'll merge this 👍