Add possibility to transfer ownership

From #239:
- The site new owner generates a new address
- You put "transfer": "newsiteaddress" to content.json
- When the clients receive that they rename the directory to "newsiteaddress" and transfer the settings (size limit, etc.), so they don't have to re-download everything

Start by tracing how clients read content.json and handle site directories. Check how the proposed transfer address, directory rename, and settings such as the size limit would be represented. Done means clients can transfer ownership and preserve existing data without downloading the site again.

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