Make "New for" column support multiple observers

[Adam Winer]

The most basic support would be following the current observer; but I think it’d be better if the ReportSetScanner ticked separate World lists for each observer, and then you could output:

• “World” if new for all observers
• Comma-separated list of abbreviations if new for the world list of only some observers

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[Adam Winer]

Resolve issue #639: Make "New for" column support multiple observers

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