When exporting conversations, only export participating users

[mootari]

Is your feature request related to a problem? Please describe.

When a specific conversation is exported via the -c argument, the users.json file will still contain the entire members directory. In popular Slacks this can easily be megabytes of data even when only a conversation between two members gets exported. This data may also contain PII that I'm not allowed to store outside of Slack.

Describe the solution you'd like

When exporting conversations, allow the export of users to be limited to participating members.

Describe alternatives you've considered

• I tried to export with the -no-user-cacheargument, but it will fail with application error: failed to create an index: empty users data base.
• I could manually read and purge the users.json, but this feels error prone.

Additional context

• I've deliberately left out mentions, because for archival purposes their user data shouldn't be relevant (I think?).
• Restricting to participants may also speed up the export, as fetching the entire user directory can take a minute. I realize though the current architecture may not support this boost.

[rusq]

TODO:

• See if the proposal from this discussion can be included #167