remove jumbotron hack that prevented sigmacanvas to go below dafixedtop and add a simple spacer to do it instead

rename leftcolumn => sidecolumn + remove its custom css from templates/graphExplorer/explorer.html to just set the width setting via settings_explorer sidecolumnSize param

Add a setting for the width of the sidecolumn + simplify resize handler sigmaLimits() (remove jquery, simplify visible height tests)

[MSG OK] better error handling between server-side graph generation and the error messages shown on client-side

Rename our server-side "graphExplorer" app to "graph", in order to prevent confusion with client-side tina-based graphExplorer from static/lib

(recuperation de la nouvelle façon d'envoyer des messages + statut HTTP pour utilisation côté graphExplorer)