This commit also moves the JobHandle into the `Gargantext.Utils.Jobs.Monad` module.

This commit renames the `Gargantext.Utils.Job.API` module to
`Gargantext.Utils.Job.Internal`, because it turns out it's *not* what
Gargantext is using directly; rather, the user-facing function in use
lives at `Gargantext.Utils.Jobs` and is called `serveJobsAPI`. This
distinction makes clearer who is using what.