Fix company submission form

[roskakori]

Currently the company submission for at /companies/submit/ is disabled in its urls.py. This form does not seem to work as intended and the test cases are broken.

It seems that currently the form can be used even without being logged in. Possibly this is a result from switching to Django 4 and some changes under the hood compared to Django 1. Maybe this can be solved with LoginRequiredMixin.

To fix this address all source code comments marked with TODO#40.