This commit splits the old `NodeAPIEndpoint` type into three;

* `NodeAPIEndpoint`, which will also contain the freeze endpoint;
* `AnnuaireAPIEndpoint`, which is the plain old node API without extra
  features;
* `CorpusAPIEndpoint`, which will also contain the publishin endpoint.

This split ensures that we don't add endpoints which do not belong to
all three categories, like before.

This commit adds a new `NodeType` called `NodeReadOnly`, to model nodes
which can be exported as read-only (frozen) snapshots. It also extends
the code to have a (provisional, in its shape) `HyperdataReadOnly`.

Previously we would fail with a generic message suggesting to run
`gargantext-init`, but that wouldn't always be the case; sometimes
there might be problems connecting to the database, and we want to
show the raw exception. This code should help diagnosing what's wrong.

It also changes the error hint to report the correct executable, as
these days we use `gargantext-cli init` instead.

For some reason the default language (namely EN) was ignored when reading
the configuration file. After discussion with anoe, it appears that
this exclusion happens for historical reasons, so this commit removes it.

Now everything is in Core/Config

`gargantext-cli ini` now takes two optional arguments:
- `--ini-path somefile.ini`: will read the configuration from `somefile.ini`.
  Defaults to `gargantext.ini`
- `--toml-path somefile.toml`: will write the configuration converted to TOML
  into `somefile.toml` (warning: it will erase the former file if there is already one!)
  Defaults to `gargantext-settings.toml`

Since it writes to a file, the command no longer outputs the configuration
to standard output.

I moved HasConfig to Core/Config instead of it being in Database.Prelude