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.