... instead of returning a `Maybe` to the frontend

Resolve "Refactor `Gargantext.Database.Prelude`"

Closes #421

See merge request !372

Those definitions felt very ad hoc, and were not used a lot in the
codebase, so I simply replaced them with their definition.

Resolve "Job workers & persistence"

See merge request !371

This reverts commit ff00345c

Maybe this will fix notifications for job finish

It might be that notifications get mixed up when running parallel
test (as they use the same port)

More import refactoring

[TESTED] Worker Tested and README info to create the database, launching and configuring it. Thank your Przemysław for this nice upgrade.