cubicweb-brainomics #3501793 Add a button to download data as Json [validation pending]

We should add a button in the download box to download data as json.

prioritynormal
typebug
done in0.8.0
load0.100
load left0.000
closed by#6a8c43ec0be1 [json] Add json download button, closes #3501793
patch[json] Add json view [applied]