----------------
- Add option to only display a summary of the migration results.
[lgraf]
- Add logging of detailed migration results to logfile (optional).
[lgraf]
- Add migration for global roles (portal_role_manager).
[lgraf]
- Add support for pre- and post-migration hooks.
[lgraf]
- Make sure `Migrations` field always uses the CheckBoxFieldWidget.
[lgraf]
- Use transaction.doom() for dry runs.
This ensures that even an accidental commit() can't result in a DB write.
[lgraf]
- Add support for programmatically providing principal mappings
by registering an IPrincipalMappingSource named adapter.
[lgraf]
- Rename `user` to `principal` where applicable:
Most of the operations work for groups as well as for users.
Therefore the mapping can contain principal IDs, not just
user IDs.
[lgraf]