Sunset private.nextflu.org

[joverlee521]

We've chatted about this multiple times over the years, but never wrote down concrete plans.
Extracting plans from latest chat about this with @huddlej in Slack:

• add titer model measurements collection to nextflu-private builds
• email CCs with plans to sunset private site (include deadline 1~6 months?)
• remove private.nextflu.org build (config, profile, GH Action workflow, deploy GH Action worklfow)
• maybe redirect private.nextflu.org -> nextstrain.org/groups/nextflu-private
• remove Netlify sites & account