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

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

Read in backend works now, insert and upsert not yet.