Request: use Ditto Theme kind for theme events

**Motivation**
This is a Nostr interoperability request for Buzz compatibility with other Nostr clients.

**Proposed solution**
Buzz switches from NIP-78 themes to Ditto's theme interchange format: https://about.ditto.pub/reference#ditto-theme

**Alternatives considered**
Making Buzz themes the standard - but I think Ditto themes are a better format, and already have adoption among Nostr clients.

**Additional context**
Ditto source code: https://gitlab.com/soapbox-pub/ditto

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Before opening: please [search open issues and PRs](https://github.com/block/buzz/issues?q=is%3Aopen) for duplicates — link the closest one, or say "none found".