Fix the project auto devops API
If `project_auto_devops.enabled` is nil for a project, when setting any auto devops values via the API, we try to create a new row in the DB, instead of re-using the existing one. This leads to the project_id being set to nil, and the database `NOT NULL` constraint leading to a 500 response. This commit resolves the issue by correctly detecting the presence of a ProjectAutoDevops row and re-using it. Persistence is also moved away from explicit `update!` calls and into relying on `autosave: true` on the model.