Copy/paste settings

# Feature request

Let the user access a text representation of their settings.

## Summary

The desired doc selection and other settings are likely to be the same across machines, browser profiles etc. A way to copy/backup/export and paste/restore/import those settings would make devdocs easier to make your own.

## Examples

Example designs for this feature:

* Put two buttons in settings, one for Copy and one for Paste. Copy will put a serialized representation of all settings into the clipboard and confirm to the user. Paste will read the clipboard, parse/validate the payload and then ask the user whether they're sure they want to overwrite settings.
* Put two buttons in settings, one for downloading a settings file and one for importing settings from a file.
* Put one button in settings that puts a setting restore link into the clipboard. If the user navigates to this URI, they are asked to confirm overwriting the settings.
* Put a textarea in settings that holds a JSON or YAML representation of all settings. All settings can be edited as normal or in the textarea, and the two representations will stay in sync.

未指定文件、测试或实现入口。首先定位设置界面以及当前设置的存储方式，然后明确导出/导入设计和哪些设置属于范围。完成内容应包括已达成一致的交互方式、验证和覆盖确认行为，以及针对成功恢复和无效恢复的测试。

由索引模型根据 Issue 内容生成。