AGPUSH-980 : Export installation + UI

This PR contains a first version of the export service + UI 
Please note that the import UI is not yet there and will be provided in a separate PR 
screencast : https://www.youtube.com/watch?v=HFXesTHh4PM&feature=youtu.be

For Master only

@edewit @matzew @abstractj  Could you review please ? 
If we export the "id" of a category, we won't be able to import that after.