[PP-1] Simplify CKEditor5ImageController once #2940383 lands

Problem/Motivation

CKEditor5ImageController duplicates a bunch of code that's already duplicated across file, rest, and jsonapi modules.

Steps to reproduce

Proposed resolution

Once #2940383: [PP-4] Unify file upload logic of REST and JSON:API lands, refactor CKEditor5ImageController to use that service.

Remaining tasks

User interface changes

API changes

Data model changes