PLT-1527 Add a slash command to set yourself away (#3752)

* added handlers for slash commands

* added manual status persistance

* added tests

* removed extra debug output and comments

* rebase - fixing the PR

* making echo messages after slash commands ephemeral

api/web_hub.go

@@ -100,7 +100,7 @@ func (h *Hub) Start() {
 				}
 
 				if !found {
-					go SetStatusOffline(userId)
+					go SetStatusOffline(userId, false)
 				}
 			case userId := <-h.invalidateUser:
 				for webCon := range h.connections {