API changes during upgrading to v30

Quick overview

Trailing slash is removed from all URLs
- e.g. /members/ is now /members
All endpoints now starth with the API version: /<str:version>/*
- e.g. /products is now /v20/products or /v30/products
/members/<str:username> is now /members/username/<str:username>
/products/<int:folder_id> is now /products/folder/<int:folder_id>
Multiple endpoints added

Overview of all URLs (rough overview, we could probably define this using OpenAPI or sth)

/streeplijst GET ping from the local server
/streeplijst/ping GET ping from the local server
/streeplijst/<str:version> GET ping from the local server
/streeplijst/<str:version>/ping GET ping from the local server
/streeplijst/<str:version>/members GET all members (not supported in v30, likely times out in v20 unless query username is used)
/streeplijst/<str:version>/members/username/<str:username> Get member by username
/streeplijst/<str:version>/members/id/<int:id> GET member by Congressus ID
/streeplijst/<str:version>/products GET all products (not supported in v30)
/streeplijst/<str:version>/products/folder/<int:folder_id> Get products in a folder
/streeplijst/<str:version>/folders GET all folders specified in the Streeplijst folder specification (stored in streeplijst.congressus.utils for now)
/streeplijst/<str:version>/sales/<str:username> GET all sales for a specific user (not supported in v20, not implemented in v30 yet)
/streeplijst/<str:version>/sales POST a new sale (not supported in v20)
- POST data should be in the following format:

{
  "member_id": <int>,
  "items": [
    {
      "product_offer_id": <int>,
      "quantity": <int>
    }
  ]
}

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