This commit makes possible to pass an optional `X-Garg-Error-Scheme`
header in a client request to enable the new error format.

This commit starts adding the conversion functions from a
`BackendInternalError` to a `FrontendError`.

It was only for test for now.

This commit tries to improve the `isSharedWith` SQL function to deal
with arbitrarily deep shared nodes in the hierarchy.

This commit introduces a new DB function called `isSharedWith`
that can be used to check for team/public/private nodes and their
shared resources.

(no default getCurrentTime anymore)

This is a first stepping stone in avoiding bugs by having `UserId` and
`ContextId` being proper types, and not synonyms.

We also refactor things to have `AuthenticatedUser` carry the `UserId`.

Basically, Gargantext.Prelude exports all of Protolude now.