User-based swatches handling

[axelveber]

As an editor, I want to pick colors from a configurable list via swatches.
It would allow me to create colored contents, while being coherent with my website color scheme.

Ideally, colors would vary by users, where each editors create their own swatches by picking a color on the picker and clicking a "Add swatch" button. A "Remove swatch" button would also allow to remove the current color from the swatches.