[server/test] stop using email cube - make unittest_migractions use a separate datadir, and add a fake email cube there (we use it to test {add,drop}_cube, so need something that looks like a cube) - for the other test modules, just copy what we need from cubicweb-email's schema to our test schema

[web/views] Remove unused variable

[server/session] bring anonymous_connection back Fixes cubicweb-comment.

[migration/3.21] idempotency fixes Drop constraints before (re)creating them This is postgresql-specific syntax, sqlserver needs work.

[migration/3.21] Make index and table creation idempotent

[migration/3.21] Add missing foreign keys when upgrading Try to delete dangling relations, since there's no point in keeping them. Related to #4846892.

[schema2sql] properly consider skip_relations parameters. This is the same bug as yams #286912. Also, introducing a unique function telling if the relation should have a table or not unhide some other problems related to computed relation, or missing skipped relations information.

[hooks/test] don't rely on RelationDefinition implicitly Its presence in globals is deprecated and going away.

[web] write css_class to attrs on rendering and not on button creation Because css_class attribute may be overriden by bootstrap after button creation and this modification has to be taken into account. closes #5457548

[statsd_logger] handle ipv6 addresses