[Admin][Settings] Introduce promotion category creation and modification capability

[chaimann]

Description

Implement the functionality to add/edit promotion categories.

We already have a promotion categories index page:

We're missing a create/edit page that utilizes the new UI components, which could replace this part in a modal component:

Features

• Interface for defining promotion category name and code in a modal component.

[fthobe]

Hey Chaimann,

thank you for your request. A migration to the new promotion system is undergoing.

Please consult following articles:
https://solidus.io/blog/solidus-v4-4 with the news about the new promotion system;
https://github.com/solidusio/solidus/blob/main/promotions/README.md with the readme.md of the new promotion system
https://github.com/solidusio/solidus/blob/main/promotions/MIGRATING.md with the migration guide

We are in the process of upgrading the documentation to account for the new promotion system. Old promotion bugs / feature requests will not be fixed.

[mamhoff]

He's talking about adding features for the new admin. Promotion categories work almost exactly the same for both systems, so I think a PR for either system will have good chances of being merged.

[chaimann]

@fthobe @mamhoff hey guys, I'm indeed working on new admin UI, PR for this issue in particular will convert existing add/edit functionality to use modal forms instead of dedicated pages (changes introduced in #6046)

and to mention, I'm picking tasks from the Solidus Admin project, so I converted a task from there into this issue, for better tracking purposes. Please let me know if this is not the desired flow, and tasks in projects should remain as "drafts" 😄