Implement generic setup import / export steps for custom styles

@ninfaj I've implement generic setup import / export steps for the custom styles (`customstyles.json`).

I've implemented a custom styles adapter for getting and setting the styles (and handling cache invalidation) and refactored the custom styles form to use it.

I've also added tests form the custom styles form to verify that I didn't break anything and I did some cleanup and translations.

See the separate commits for details.