Draft: [server/migractions] simplify the Migration Handler entry point
There is no need to have several ways to create a migration handler. We move .set_cnx to cwctl.admincnx, which given a repository, will return an admin cnx.
Moreover, we use the repoapi.get_repository to get repositories.
Related to https://www.cubicweb.org/3933480
Edited by Nicolas Chauvat