Scope and purpose

Resolves #3537.

This PR adds closing behaviour controls to the render_modal utility. This is necessary to support "persistent modal" patterns, specifically the feedback modal in the Portadmin tool.

I have added two new options when rendering modals:

• close_on_outside_click (default: True): Whether to close the modal when clicking the background (modal overlay).
• show_close_button (default: True): Whether to show the x button in the top right corner.

This pull request

• Support close behaviour options in render_modal
• Updates tests to include new close options
• Updates styleguide to include new close options

How to test

I tested the new options by modifying the Image Upload Help modal. However, this PR does not commit any changes to existing modals as it is purely a "utility update".

To manually test the changes yourself, find a random modal and set the two new options to False.

Contributor Checklist

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• Added a changelog fragment for towncrier
• Added/amended tests for new/changed code
• Added/changed documentation
• Linted/formatted the code with ruff, easiest by using pre-commit
• The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See https://cbea.ms/git-commit/
• This pull request is based on the correct upstream branch: For a patch/bugfix affecting the latest stable version, it should be based on that version's branch (<major>.<minor>.x). For a new feature or other additions, it should be based on master.
• If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done
• If this results in changes in the UI: Added screenshots of the before and after
• If this adds a new Python source code file: Added the boilerplate header to that file