GN-5309: add support for fusie IVs

[elpoelma]

Overview

This PR includes support for 'fusiegemeenten' to set up their inauguration meeting.
Specifically, the following changes are included:

• A slight rework of the installatievergadering api for readability. Includes a custom Map implementation with support for default values and batch-setting map entries
• The addition of the fusietemplates to the IV configuration

connected issues and PRs:

GN-5309

Setup

Ensure that you're connected to QA/PROD RB

How to test/reproduce

• Start the app
• Login as a 'fusiegemeente' gemeente/OCMW
• Ensure you can create both a 'gemeente' and 'OCMW' inauguration meeting
• You should still be able to create meetings in other municipalities
• Both 'Lokeren' and 'Antwerpen' should have no 'Politieraad' AP

Challenges/uncertainties

I reworked the installatievergadering API to what I think is a bit more readable/less dense. Let me know if you'd prefer the old version.

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