Also, fix for notifications throttle delay value

This reverts commit f4284b4e.

This commit refactors ever so slightly the '.Stem' modules to create a
unified interface; now Gargantext.Core.Text.Terms.Mono.Stem exports a
single 'stem' function which can be used with different
`StemmingAlgorithm`s, and now the other algorithms' functions have been
shielded behind internal modules.

This makes possible to see at glance which part of Garg are using which
stemming algorithm, as it wasn't always clearer before.

This also allows us to have a better control over the langugages, as not
all the stemming algorithms supports all the languages that Garg
supports.

This reverts commit efbe327f, reversing
changes made to 6d9bcd07.

We have contexts.hash_id which is enough (computed by postgres as a
trigger).

When user searched for a phrase with space inside, in database, the
'to_tsquery' function raised an sql error. One should use
'plainto_tsquery'. Also, a proper test was added.

- DBCmd instead of Cmd
- remove FlowCmdM in favor of HasNodeStory

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.