Allow for custom palettes to be defined

[dero]

Feature description

As a user, I want to be able to add certain colors to a "Custom" color palette and select this palette when creating/editing a story alongside the predefined palettes.

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

Dependent on #3067 #3068

• AC1: Users are able to add and remove custom colors to a custom color palette.

Implementation brief

QA testing instructions

Demo

Changelog entry

[dero]

@MackenzieHartung, I've created this issue per request from @swissspidy. Could you please make sure all labels and other things are assigned correctly?

[MackenzieHartung]

@dero all immediate labels are there. The two we will need to add are ticket size and sprint. Is this intended for Sprint 15? If so, I can add that label. Additionally, if you will be working on this just please add an estimation size when you begin work.

[dero]

@MackenzieHartung Thank you. And yes, unless we de-prioritize the feature, this is intended for Sprint 15.