Make the language specific favicon optional

Would it be possible to make the language specific favicon optional? I use only one instance and the new colorful favicons are a distraction. E.g.:

> - [x] Enable language specific favicons

_Originally posted by @mrflix in https://github.com/freeCodeCamp/devdocs/issues/803#issuecomment-526491644_

首先定位语言特定 favicon 的选择位置以及用户选项的配置位置。跟踪现有的“Enable language specific favicons”行为，然后确定禁用它应如何影响单实例安装，并验证最终的 favicon 行为。

由索引模型根据 Issue 内容生成。