cubicweb-conference #1381229 Add an url_id attribute to Conference to have a unique url identifier [validation pending]

Add a unique identifiant for conference entities to backport url rewriting in conference

Notes from the patch:

Modify the fsschema to avoid conflict with the two constraints unique=True and required=True (as required=True, column is filled with the same null value, so the constraint unique=True fails).
prioritynormal
typeenhancement
done in0.11.0
load left0.000
closed by#17c99b4077da add attribute url_id to Conference to have a unique url identifier (closes #1381229)