Scales - please let us import/export config

## Description
It would be useful to be able to re-visit previously set-up colour scales either via exposing the parameters in the URL (like [this app](https://andrewringler.github.io/palettes/#/9|s|00429d,96ffea,ffffe0||1|1|0|) does) or have some other way of exporting and importing configurations.

## Why do you need this feature?
I'm working with multiple colour gradients for my organisation, and depending on the specific use case I need the hex codes for varying amounts of stops on them. The way the app currently is, I need to re-input all the key colours and all other config options each time I open the app. Implementing a config export/import feature would save me a lot of hassle.

The issue names no files, tests, or entry points. Start by tracing how the app stores colour-scale controls and configuration, then compare URL-based persistence with import/export requirements. Done means users can revisit configurations and obtain the requested hex codes without re-entering the scale settings.

Written by the indexing model from the issue text.