Also, use genericArbitrary in Instances, this simplifies things

Also, some import refactoring

This function extracts the Node.Update logic for `publish_node` into a
separate function, so that we can use it in a `POST` request, i.e. a new
endpoint for publishing nodes.

Previously there was no symmetry in the name of the share endpoint,
which is meant to be used to generate a publicly-accessible URL to view
a node, and the previously-called "unpulish" endpoint.

The problem is that now the "unpublish" name would clash with the dual
of publishing a node. To avoid the problem, this commit renames the old
"unpublish" endpoint to be "unshare", so that now it's clear that is the
inverse operation of "share".

This allows the frontend to correctly display a different icon in case a
node has been published with edits enabled or not.

Previous to this commit, `get_user_root_node_folder` would fail with
a `UserHasTooManyRoots` in case this was called on a tree which had
more than one "public" folder.

We fix this by taking the oldest available node, which will always
correspond to the notion of "root", being the first ever created.

Ref: #238, #341, #418

Ref: #238, #341, #418

Also added a warning in `.ghci` in the form of a comment discouraging
people from overriding local REPL configuration options

This commit splits the policy checks we do on node operations into
"read" checks and "write" checks, so that we can enforce different kind
of policies based on the state of a node (i.e. published or not).

This commit refactors the `NodeNodeCategory` so that we create a better
hierarchy to account for node publication, including all the policies it
might have.

It also:

* Removes UserNodePublic from typesWhiteList in isNodeReadOnly

This commit refactors a bit the internal of the
`Gargantext.Database.Query.Tree` module so that it removes a lot of
repetitive boilerplate and paves the way to more interesting features.

It also:

* Adds the `isUserNode` boolean query
* Adjust source and target in publishNode and unpublishNode
* Pass the currently-logged-in user to tree API functions

It also:

* Amends tests for policy checks status code changes
* Fix a bug in isNodeReadOnly
* Adds a function to return all the nodes ids for the published
  nodes. Next we need to modify `findShared` & co to make sure that
  published notes will show somewhere in the users' tree.

Now this query accounts correctly for recursive children.

It also:

* Adds basic publish/unpublish works
* Add Move API tests scaffolding

Now a `NodeNodePoly` stores a typed `NodeNodeCategory`, we can use
to enforce invariants and making lookups more efficient.

This commit makes this function private to the `Table.NodeNode` module
as it's fairly low-level, and it doesn't let us embed logic to ensure we
are creating a valid node2node relationship. Better use one of the other
specialised functions.