[#5616] reduce slider arrow size

[hom3mad3]

Describe your changes fixes https://github.com/liqd/a4-meinberlin/issues/5616

Tasks

• [ ] PR name contains story or task reference
• [ ] Steps to recreate and test the changes
• [ ] Documentation (docs and inline)
• [ ] Tests (including n+1 and django_assert_num_queries where applicable)
• [ ] Changelog

[goapunk]

The buttons and the numbers now overlap for me: