It forces programmers to think about errors we are logging and reporting
to the frontend, because they need to contain no sensitive data.

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.

Basically, Gargantext.Prelude exports all of Protolude now.

Also, use ISO639 languages in some places.

This reverts merge request !166

This reverts commit b3fb1a16

It introduced changes which cause issues described in
gargantext/purescript-gargantext#571

It's not fully wired in, `convertQuery` for Arxiv is implemented as
`undefined`.

This commit simplifies the `ExternalAPIs` type, which is now a pure
enumeration, moving the lookup of the Pubmed key at the usage sites.
This simplified the code quite a bit, and made some otherwise-effectful
functions pure.

NOTE This doesn't compile yet.

This still isn't optimal because in our code we use lists, so this
will fetch everything (using conduit), coerce into a list and then
process.

This turned out to be quite heavy because of module import cycles.