This beefy commit removes the old (unnamed) Servant API.

This big commit ties the knot with the named _routes_ by providing
concrete server (i.e. handlers) implementations.

Merge branch '345-docs-tsv-export-import-check-and-maintain-consistency-between-export-and-import-formats' of https://gitlab.iscpif.fr/gargantext/haskell-gargantext into 345-docs-tsv-export-import-check-and-maintain-consistency-between-export-and-import-formats