mapstore2 extensions folder and file

[gaetanbrl]

The mapstore/README.md contains a extensions.json detail :

extensions.json: dynamic registry of currently installed extensions, in JSON format

But we have nothing about that when we clone the datadir.
From last release, we have a new folder extensions to contains extensions.json file.

Btw, I think that we could provide this folder and a ready extensions.json file to avoid default install error.

I will create a PR next.

[Landry Breuil]

makes sense, thanks !