SeedDB bulk import file upload crashes

[lunkwill42]

Describe the bug
Attempting to upload a file to bulk import seed data under NAV 5.0.0 crashes the SeedDB page with an AttributeError (which is masked by a broken custom 500 error handler view, which @hmpf is working on separately).

To Reproduce
Steps to reproduce the behavior:

1. Create an organization bulk import file, containing only a single line, e.g. foobar
2. Go to the SeedDB in the NAV web ui
3. Click on Organization -> Bulk Import -> Choose file
4. Select the bulk import file you created.
5. Click Preview import.
6. See error

Expected behavior

A preview of the bulk import results, as would happen if the import data was entered manually into the text area below the Choose file button.

Environment (please complete the following information):

• OS on the NAV server: Debian 10
• NAV 5.0.0
• Method of installation: Debian package

Additional context

This is the logged traceback:

Internal Server Error: /seeddb/organization/bulk/
Traceback (most recent call last):
  File "/opt/venvs/nav/lib/python3.7/site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/opt/venvs/nav/lib/python3.7/site-packages/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/opt/venvs/nav/lib/python3.7/site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/web/seeddb/page/organization.py", line 92, in organization_bulk
    extra_context=info.template_context)
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/web/seeddb/utils/bulk.py", line 34, in render_bulkimport
    form = BulkImportForm(parser_cls, request.POST, request.FILES)
  File "/opt/venvs/nav/lib/python3.7/site-packages/nav/web/seeddb/forms/bulk.py", line 45, in __init__
    self.data['bulk_data'] = bulk_file.read()
  File "/opt/venvs/nav/lib/python3.7/site-packages/django/http/request.py", line 435, in __setitem__
    self._assert_mutable()
  File "/opt/venvs/nav/lib/python3.7/site-packages/django/http/request.py", line 432, in _assert_mutable
    raise AttributeError("This QueryDict instance is immutable")
AttributeError: This QueryDict instance is immutable