This commit adds a `panicTrace` function to be used as a drop-in
replacement of the `panic` function, with the difference that we
wrap the error in a `WithStacktrace` error, to be later caught
and logged.

Also, some fixes to NodeStory (it inserts ngrams when saving node
story, when they don't exist).

Basically, Gargantext.Prelude exports all of Protolude now.

Tests were written with a very flat module hierarchy which made them
a bit confusing to navigate. This commit moves everything under a common
`Test.*` hierarchy.

We now require CoreNLP listening on port :9000. We will have to find
a way to spin that up in CI as well.