Refactor form views

[JoeyStk]

Short description

This PR is based off of #3837 and can only be reviewed (properly) after the Django update is open :). During our work on the upgrade, we had to do a few changes to the poi_form_view and event_form_view. Those changes increased the complexity of the files (for poi_form_view it even made it necessary to disable a ruff rule). This PR attempts to fix these issues.

For an easier view to review this, this link might be helpful :)

--- Info ---
This PR was handed over to @MizukiTemma

Proposed changes

• Reduce CC of event_form_view from 30 to 26
• Reduce LOC of event_form_view from 201 to 177
• Reduce CC of poi_form_view from 33 to 14
• Reduce LOC of poi_form_view from 232 to 141
• Reduce statements in poi_form_view, so that rule can be enabled again

Side effects

• This refactoring is by no means "perfect", these were just a few low hanging fruits I wanted to collect for us.
• Feel free to either extend this refactoring or refactor these files even more in the future
• As with all refactorings, I tried my best to pay attention to all the details, but of course there is a high chance of human error, so horough testing is highly appreciated.

Faithfulness to issue description and design

There are no intended deviations from the issue and design.

How to test

• All actions in event form and poi form :)

Resolved issues

Fixes: #4208

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Pull Request Review Guidelines

[MizukiTemma]

Waiting for #3837