refactor: Simplify border point implementation

[MoritzWeber0]

• Border points are expected to be between exactly two countries. There was just one case where more than two countries were used: Eurostar on the UK page. Since Eurostar didn't show the list of countries on the Germany page either, I removed the option to include multiple countries. Replaced with country France, all Eurostar services from UK pass France anyway.
• Add a new shortName to operators and use it as label for border points. For consistency, I changed LTG-Link to LTG, Hellenic Train to HT and ZSSK / ZSR to ZSSK as short names.
• Render HTML directly instead of Markdown, move implementation to own folder. More fail-safe than the generation of markdown tables.
• Move the border points selection via IDs to the corresponding countries front matter files. Only the border point definitions stay in the border points file.
• Properly register the border points file as singleton in the CMS. Add relations from the border points definition on the country pages to the values of the singelton, include a link where to edit it. Add a border points editor component.
• Removed the sort attribute since we have full-control over the sorting in the border points file. Performed a one-time sorting on the element where sort: true was set.
• Since the variant attribute of the expander shortcode is no longer needed (faq and border points have their own handling), I replaced variant with icon to allow more flexibility.

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