added check for "copy_relations"

this check tests if all CMSPlugins that have an m2m or a reverse fk define a
"copy_relation" method.

The check only outputs warnings, since there may be good reasons to omit
the copying of related data.
added some docs to the 2.4 upgrade guide for the copy_relations check
use plugin_pool instead of CMSPlugin.__subclasses__() to discover plugins
Merge pull request #1710 from piquadrat/feature/copy_relations-check

added check for "copy_relations"