增加了一个超链聊天跳转

在“运营设置里面”增加了“聊天页面2链接”，方便将项目（https://github.com/Dooy/chatgpt-web-midjourney-proxy） 替换掉原来的AMA问天。

Changes to be committed:
    modified:   common/constants.go
    modified:   controller/misc.go
    modified:   model/option.go
    modified:   web/src/App.js
    modified:   web/src/components/OperationSetting.js
    modified:   web/src/components/TokensTable.js

common/constants.go

@@ -21,6 +21,7 @@ var Footer = ""
 var Logo = ""
 var TopUpLink = ""
 var ChatLink = ""
+var ChatLink2 = ""
 var QuotaPerUnit = 500 * 1000.0 // $0.002 / 1K tokens
 var DisplayInCurrencyEnabled = true
 var DisplayTokenStatEnabled = true