Resolve "Fix test build warnings"

See merge request !376

Also, add -Werror to tests so that no warnings in tests are reported anymore.

Switch to OpenAPI3 from Swagger2, to use proper Bearer token

Closes #427

See merge request !369

This commit adds three major tags to the swagger UI:

* Gargantext (like before): this spells out all the endpoints;
* Private operations: lists private backend ops;
* Public  operations: lists public backend ops.

More can be added, but the machinery used here should serve as a
blueprint.

OpenAPI3 will allow us to properly pass the Bearer Auth to the servant
swagger UI.

I fixed those warnings that were easy to fix: redundant imports, unused
variables, etc. (I might have missed some though)

The warnings I deliberately left for now are:
* In `Test.API.UpdateList`: Incomplete pattern matching (we would need
  to think a bit about error handling to fix this)
* In `Test.Utils`: `traceShowId` remains in code (we would need to think
  a bit about proper logging to fix this)

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