[MM-17422] Added code to update, delete last bot_icon time in model.bot. (#12229)

Display LHS bot Icon in web app. As part of mentioned task, Added LastIconUpdate variable in model.bot to store last update time of icon. Also added code to update/delete value of the mentioned variable when setting/deleting bot icon.

model/bot.go

@@ -22,14 +22,15 @@ const (
 // Note that the primary key of a bot is the UserId, and matches the primary key of the
 // corresponding user.
 type Bot struct {
-	UserId      string `json:"user_id"`
-	Username    string `json:"username"`
-	DisplayName string `json:"display_name,omitempty"`
-	Description string `json:"description,omitempty"`
-	OwnerId     string `json:"owner_id"`
-	CreateAt    int64  `json:"create_at"`
-	UpdateAt    int64  `json:"update_at"`
-	DeleteAt    int64  `json:"delete_at"`
+	UserId         string `json:"user_id"`
+	Username       string `json:"username"`
+	DisplayName    string `json:"display_name,omitempty"`
+	Description    string `json:"description,omitempty"`
+	OwnerId        string `json:"owner_id"`
+	LastIconUpdate int64  `json:"last_icon_update,omitempty"`
+	CreateAt       int64  `json:"create_at"`
+	UpdateAt       int64  `json:"update_at"`
+	DeleteAt       int64  `json:"delete_at"`
 }
 
 // BotPatch is a description of what fields to update on an existing bot.