MM-56579: Include deactivated users from DM posts (#25985)

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>

server/channels/store/sqlstore/post_store.go

@@ -2686,7 +2686,7 @@ func (s *SqlPostStore) GetRepliesForExport(rootId string) ([]*model.ReplyForExpo
 	return posts, nil
 }
 
-func (s *SqlPostStore) GetDirectPostParentsForExportAfter(limit int, afterId string) ([]*model.DirectPostForExport, error) {
+func (s *SqlPostStore) GetDirectPostParentsForExportAfter(limit int, afterId string, includeArchivedChannels bool) ([]*model.DirectPostForExport, error) {
 	query := s.getQueryBuilder().
 		Select("p.*", "Users.Username as User").
 		From("Posts p").
@@ -2696,13 +2696,17 @@ func (s *SqlPostStore) GetDirectPostParentsForExportAfter(limit int, afterId str
 			sq.Gt{"p.Id": afterId},
 			sq.Eq{"p.RootId": ""},
 			sq.Eq{"p.DeleteAt": 0},
-			sq.Eq{"Channels.DeleteAt": 0},
-			sq.Eq{"Users.DeleteAt": 0},
 			sq.Eq{"Channels.Type": []model.ChannelType{model.ChannelTypeDirect, model.ChannelTypeGroup}},
 		}).
 		OrderBy("p.Id").
 		Limit(uint64(limit))
 
+	if !includeArchivedChannels {
+		query = query.Where(
+			sq.Eq{"Channels.DeleteAt": 0},
+		)
+	}
+
 	queryString, args, err := query.ToSql()
 	if err != nil {
 		return nil, errors.Wrap(err, "post_tosql")