Categories listing should read from internal endpoint, not fixture

Problem/Motivation

Currently, the category list rendered on the left is created by reading from "fixtures/category_list.json" directly. The code in the svelte file is:
import category_list from '../../fixtures/category_list.json';

We should not read from a fixture, instead we should read from the internal endpoint which will read from the fixture or from d.org (depending on the plugin that is in use). The endpoint is "/drupal-org-proxy/categories".

Proposed resolution

Change the svelte logic to read from the endpoint.

Remaining tasks

• ✅ Create internal endpoint
• ☐ Use that endpoint in svelte

User interface changes

None.

API changes

Yes, create a new endpoint. UPDATE: this is done already and in the main branch.

Data model changes

None.

Release notes snippet

Add and consume endpoint to get the available categories.