IV LMB queries: add support for 'district' bestuurseenheden

[elpoelma]

Overview

This PR adds support to the LMB queries for 'district' bestuurseenheden.
Specifically, the 'gemeenteraads' queries are adjusted to also work for 'districten'.

connected issues and PRs:

None

Setup

• Set-up the backend
• Ensure the ldes-client service has run

How to test/reproduce

• Quite difficult as the 'Opstart nieuwe legislatuur' feature is not yet enabled for 'district' bestuurseenheden on LMB
• Start the frontend
• Log in as a 'district' (e.g. 'Berchem')
• Create an IV
• The sync should run without errors

Challenges/uncertainties

The implementation is a bit hacky, as we just use a simple object to look up the correct 'classificatie-codes' for 'districten'/'gemeentes'.

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check cancel/go-back flows
• Check database state correct when deleting/updating (especially regarding relationships)
• changelog
• npm lint
• no new deprecations