[pkg] prepare 3.20 release

[skeleton] simplify debian rules file

[migration] don't crash when deleting a non-existing rtype When asked to delete a relation type, instead of checking if it was computed or not, just delete both the CWRType and CWComputedRType.

[debian] include "metapackage" in the cubicweb package's description Lets lintian know it's ok for it to be empty.

[debian] ensure all packages use ${misc:Depends} and ${python:Depends}

[debian] update cubicweb-dev's lintian override Switching from python-support to dh-python changed the installation path.

[server] add a db-namespace option in source definition (closes #1631339) "namespace" is preferred to the postgresql term "schema" to try and avoid confusion with cubicweb's schema. cubicweb now depends on logilab-database >= 1.13.0

[serverctl] add db_transaction context manager

[server] refactor cleanup() code used by "cubicweb-ctl delete" Try to make each logical step (i.e. "delete database" and "delete user") a bit more visible and explicit. This is a preparatory work to easily plug the "delete namespace" feature.

[views] visual refactorings (closes #3803685) - pageContent: - remove the shadow - add a light border - slightly round the corners - left column: - ensure width make it aligned with the logo (header) - ensure the top border of the first box is aligned with the pageContent div - do not use serif font for box headers