Removing the `All` data constructor from  the `Lang` datatype has
several benifits, the main one being that we now have total functions
for things like `toISO693`.

This will make possible to write an uniform interface for stemming
`stem :: Lang -> StemmingAlgorithm -> T.Text -> T.Text`, as now
`Lang` will always map to a valid (ISO693) language.

This allows us to have only one single (static) map which will be
treated by GHC as a CAF.

Basically, Gargantext.Prelude exports all of Protolude now.

Also, use ISO639 languages in some places.

Also, added a basic GraphQL endpoint.