Abstract TZ picker into package

[jonthegeek]

The code for detecting user timezone and allowing them to override that detection is useful. Pull it off into a package and publish.

Strongly consider using tzdb::tzdb_names() rather than OlsonNames() for future-proofing (but requires tzdb/clock for time stuff in that case!). Probably abstract that part out so the places that reference olsonNames() instead call something that knows what this server is using as the source of tz truth.

Make the drop-down optional. Be sure to also allow/support styling.

Ping Eric about it, since it's inspired by his code originally.

Anything else?