Refactor `post` in `integreat_cms/cms/views/pois/poi_form_view.py`

[MizukiTemma]

Background and current state

The post function in integreat_cms/cms/views/pois/poi_form_view.py has received an warning "too-many-statements (PLR0915)" by ruff. It's currently suppressed (to separate refactoring from #4136 ).

ToDo List

• Remove suppression
• Refactor post to reduce statements

Screenshot

Additional Context

#4136 (comment)

Wait for #3837

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