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

model/command_args.go

@@ -0,0 +1,36 @@
+// Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"encoding/json"
+	"io"
+)
+
+type CommandArgs struct {
+	ChannelId string `json:"channel_id"`
+	RootId    string `json:"root_id"`
+	ParentId  string `json:"parent_id"`
+	Command   string `json:"command"`
+}
+
+func (o *CommandArgs) ToJson() string {
+	b, err := json.Marshal(o)
+	if err != nil {
+		return ""
+	} else {
+		return string(b)
+	}
+}
+
+func CommandArgsFromJson(data io.Reader) *CommandArgs {
+	decoder := json.NewDecoder(data)
+	var o CommandArgs
+	err := decoder.Decode(&o)
+	if err == nil {
+		return &o
+	} else {
+		return nil
+	}
+}