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.

No file, test, or implementation entry point is named. Start by locating the settings screen and how settings are currently stored, then clarify which export/import design and settings are in scope. Done should include an agreed interaction, validation and overwrite-confirmation behavior, plus tests for successful and invalid restores.

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