cfg: add sitename variable

[tmorrell]

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Description

Add the SITENAME variable, which is used in the header and now UI elements from https://github.com/inveniosoftware/invenio-rdm-records/pull/1641.

Checklist

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

• [ ] I'm aware of the code of conduct.
• [ ] I've created logical separate commits and followed the commit message format.
• [ ] I've added relevant test cases.
• [ ] I've added relevant documentation.
• [ ] I've marked translation strings (for relevant code).
• [ ] I've followed the CSS/JS and React guidelines (for relevant code).
• [ ] I've followed the web accessibility guidelines (for relevant code).
• [ ] I've followed the user interface guidelines (for relevant code).
• [ ] I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• [ ] I've NOT included third-party code (copy/pasted source code or new dependencies).

Third-party code

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