people: remove broken Gitter chat links

[ptamarit]

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Description

The "Chat with me" links in the People page are broken and we do not use Gitter anymore. This pull request removes the chat field from the person model, from the content, and from the template.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• [x] I'm aware of the code of conduct.
• [x] I've created logical separate commits and followed the commit message format.
• [x] I've added relevant test cases.
• [x] I've added relevant documentation.
• [x] I've marked translation strings.
• [x] I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• [x] I've NOT included third-party code (copy/pasted source code or new dependencies).
  • If you have added third-party code (copy/pasted or new dependencies), please reach out to an architect.

Frontend

• [x] I've followed the CSS/JS and React guidelines.
• [x] I've followed the web accessibility guidelines.
• [x] I've followed the user interface guidelines.

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