mosaic #290589 add a variable to specify base-path [validation pending]

So that we can serve mosaic from http://demo.logilab.fr/mosaic

prioritynormal
typebug
done in0.1.1
load0.250
load left0.000
closed by#9e2ae8a1a773 server: add base path management (closes #290589)
patchserver: add base path management (closes #290589) [applied]