UX: improve new community page

[carlinmack]

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Description

Please describe briefly your pull request.

1. Cast slug to lowercase to communicate to users what the URL will actually look like
2. Improve the error message

Before and after (changes highlighted with arrows):

Checklist

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• [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.
• [ ] 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).
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Frontend

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

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