[TASK] Ease management of custom projects

This change allows deletion of custom projects and maangement
of projects' properties without manually editing sphinx-projects.json.

Change-Id: I6e5b391f8365d2603df54313ecbf600fd774fee7
Resolves: #57321
Reviewed-on: https://review.typo3.org/28942
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers