Use a form for page_size in incident list

[hmpf]

Scope and purpose

Fixes #1382

Contributor Checklist

Every pull request should have this checklist filled out, no matter how small it is. More information about contributing to Argus can be found in the Development docs.

• [ ] Added a changelog fragment for towncrier
• [ ] Added/amended tests for new/changed code
• [x] Linted/formatted the code with ruff and djLint, easiest by using pre-commit
• [x] 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 our how-to
• [ ] If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done