I want to understand ngrams algorithms better.

Also, add -Werror to tests so that no warnings in tests are reported anymore.

I fixed those warnings that were easy to fix: redundant imports, unused
variables, etc. (I might have missed some though)

The warnings I deliberately left for now are:
* In `Test.API.UpdateList`: Incomplete pattern matching (we would need
  to think a bit about error handling to fix this)
* In `Test.Utils`: `traceShowId` remains in code (we would need to think
  a bit about proper logging to fix this)

It might be that notifications get mixed up when running parallel
test (as they use the same port)

More import refactoring

Also, use genericArbitrary in Instances, this simplifies things

Also, some import refactoring

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

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.

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