PLT-1598 Slash command works in RHS (#4367)

* PLT-1598 Slash command works in RHS

* fix UserProfile in the reply for Slash Command

* fix some problem about the system messages in RHS
* system message in RHS isn't displayed as comment for root message

* remove status indicator for system message in RHS

* system message in RHS is colored to grey

* system messages don't count as commented post

* fix bug about cleaning draft in RHS

* remove unnecessary function

* implement new model for executing command

api/command_shortcuts.go

@@ -35,7 +35,7 @@ func (me *ShortcutsProvider) GetCommand(c *Context) *model.Command {
 	}
 }
 
-func (me *ShortcutsProvider) DoCommand(c *Context, channelId string, message string) *model.CommandResponse {
+func (me *ShortcutsProvider) DoCommand(c *Context, args *model.CommandArgs, message string) *model.CommandResponse {
 	shortcutIds := [4]string{
 		"api.command_shortcuts.nav",
 		"api.command_shortcuts.files",