Does anyone have an insight into the differences between Slack, Mattermost, and Gitter? I have only ever used Slack before but it seems like Gitlab is already heavily invested with Mattermost. That makes me wonder what the future looks like for both Mattermost and Gitter. Are they different enough that they can both coexist without hurting either product or is one of them destined to be folded into the other in hopes of taking on Slack more directly?