There were 2 separate bugs here:
1. We were not including deactivated users while fetching posts from
DM channels.
2. We were not respecting the includeArchivedChannels flag while
fetching DM channels.
We fix both of these issues here.
```release-note
Include posts from deactivated users in DM channel export. Also
respect the --include-archived-channels flag for DM channels.
```
https://mattermost.atlassian.net/browse/MM-56579
Co-authored-by: Mattermost Build <build@mattermost.com>