Fetch slug list from API

[knuhol]

The sitemap is currently generated from scripts/slugs.json file because of missing API endpoint. Once endpoint is added, change scripts/sitemap-generator.js script to fetch it from API instead.

[knuhol]

@terhol if you want to work on this ticket, these are things good to know:

Prerequisites

• assign yourself as working on this task here on GitHub and put the issue as in progress
• checkout the project from GitHub and install dependencies by running a command npm i
• there is npm script called sitemap, you can run it by command npm run sitemap - it will fail for now, check the further steps to fix it
• you have to set up environment variable PUBLIC_HTML, on MacOS you can do it by command export PUBLIC_HTML=https://dkdb.cz
• the sitemap file will be crated inside build folder, if it doesn't exist yet on your machine, you need to create it by command mkdir build
• once generated, the sitemap will be located in file build/sitemap.xml file which you can format in IntelliJ IDEA to a human readable format by selecting Code > Reformat file... in IntelliJ IDEA menu to check it has been generated correctly

The task

• remove scripts/slugs.json file
• in scripts/sitemap-generator.js remove the code which reads slugs from the removed file and change it to the endpoint instead (you can hardcode the address for now)
• use Fetch API to fetch the JSON from the endpoint - this is an async operation so you will need promises
• in case of error, log the error to standard error output with function console.error

Committing the change

• don't use IntelliJ IDEA git tools, prefer git CLI instead
• create a new branch by command git checkout -b fetch-slug-list-from-api
• stage changes by command git add -A
• run command npm run commit which will trigger commit helper
• select fix because of semver and automatic changelog/releases generation
• the short description asked during the committing is the commit subject (the commit message which you usually provide)
• everything else can be left empty
• once all checks pass, you can push the new branch to GitHub by command git push -u origin head (which will also track the new remote branch in case some changes needed later)
• go to GitHub, open new PR with a description that the PR closes this issue and assign me as a reviewer