Flush log and exit after CLI Slack Import. (#4286)

Some log messages were previously being lost at the very end of the
Slack import via CLI process, due to not flushing the logs after
completing.

mattermost.go

@@ -1473,6 +1473,8 @@ func cmdSlackImport() {
 		fmt.Fprintln(os.Stdout, "Running Slack Import. This may take a long time for large teams or teams with many messages.")
 
 		api.SlackImport(fileReader, fileInfo.Size(), team.Id)
+
+		flushLogAndExit(0)
 	}
 }