DEV: Experimenting with using built in core color palettes for the color picking component (#71)

This PR is the beginning of converting the color picker to use built in
core color palettes.
The `color_schemes` defined in `about.json` are created in core when the
theme is imported,
and we then show all user-selectable color palettes in this sidebar
footer menu.

An important caveat here is that all of the Horizon themes must be
changed to `user_selectable`
otherwise they will not show up in the color palette selector in the
sidebar.

When choosing a color palette that also has a corresponding dark color
palette,
*both* light mode and dark mode are correctly saved with the color
palette(s) chosen,
using the color palette cookie we already have in core.

Anon users can also set a palette, which will be saved in a cookie.

---------

Co-authored-by: Sérgio Saquetim <saquetim@discourse.org>
Co-authored-by: Martin Brennan <martin@discourse.org>
Co-authored-by: Osama Sayegh <asooomaasoooma90@gmail.com>

common/common.scss

@@ -2,7 +2,6 @@
 @import "buttons";
 @import "chat";
 @import "color-choice";
-@import "color-exploration";
 @import "composer";
 @import "header";
 @import "hiddenstuff";