Fixes #258

This commit fixes the issue of the "empty Ngrams". The culprit was a bug
in the `arxiv-api` package, which was fixed by:

gargantext/crawlers/arxiv-api!2 (commits)

In a nutshell, due to the fact the relevant `Conduit` was never
completing, it was blocking the `flow` function, which was endlessly
waiting for the last part of the results to arrive.

Hence we were never calling the `flowCorpusUser`, which is the function
responsible for generating the Ngrams.

This important commit introduces the 'DbCmd' constraint set, allowing DB
functions to require only a connection pool and some `GargConfig`.

This is to prevent accidental updates of hyperdata with other JSON
structs.

This reverts merge request !166

This reverts commit b3fb1a16

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

This reverts commit a444cb30.

This reverts commit 22b14f56.

This reverts commit d04cf03b, reversing
changes made to e213a4bf.

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.

This is safer than type Limit = Int

They are changed to MaybeFields ...Read instead when joining.

Remove joins, use optionalRestrict instead.

Also, remove cabal.project because stack.yaml and package.yaml cover
dependencies already.

Based on the suggestions in
https://kodimensional.dev/space-leak