Refactor migrate_to_sqlalchemy

- separate top-level package, indico_zodbimport
- proper cli command:indico-zodbimport
- additional migration modules can be registered using entry points

Usage is e.g. like this for RB migration:

indico-zodbimport postgresql:///indico zeo://127.0.0.1:9675 roombooking --rb-zodb-uri ... --photo-path ...