allow any pipeline configs + PEP8

From the Changelog:

- Allow any pipeline configuration to be imported through the web at the
  ``@@mr.migrator`` view. Previously, only those pipelines were shown, which
  had a ``plone.app.transmogrifier.atschemaupdater`` blueprint included. Now
  any pipelines can be used, e.g. those which handle only Dexterity objects.

- Pep8.