Added the possibility to allow or not the members
of an AnObj to publish their annotations for
the other members. Read/write support from
AnObjMgr

AnObj sharing and permission at Backend level
AnObjMgr in FrontEnd allow editing of the sharing
properties. Is able to liste shared AnObj

a field image_url added to AnObj
Image is loaded by a img tag with "crossorigin"
attribute set to "anonymous".
The paper.Raster object is build using the img tag
instead of the direct url. Also for local images.

Functions for Thumbnail creation and borders are 
now in adim_app.utils

Thumbnail creation and border are called at view
level from adim_app (no more in models)

Thumbnail size in settings, no more hardcoded