fix schema migration hooks (partial)
SQL Server requires some additional steps to alter columns
(index drop and restore, etc...) and this in turn requires
passing a connection on the DB to find out the indexes.
Also partially fix an issue where the null_allowed value was
not correctly computed. Still some problems in there
(see https://www.logilab.net/cwo/ticket/1109558)
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/shared/data usr/share/cubicweb/cubes/shared
debian/tmp/usr/share/cubicweb/cubes/shared/wdoc usr/share/cubicweb/cubes/shared