migrate 7.x-2.8-beta1

Migrate 2.8 beta 1
==================

Features and enhancements
- #2379289 - Better handle interaction of --update with highwater marks.
- #2403643 - Support an additional level of subfields.
- #2472045 - Add language subfields only if field is translatable.
- #2474809 - Provide better message for bad dependencies.
- #2397791 - Provide detailed field validation errors.
- #2309563 - Add support for running migrations via wildcard name.
- #2095841 - Abstract mail system disablement for more flexibility.
- #2419373 - Provide ability to cache map lookups.
- #2141687 - Provide detailed message on file copy error.

Bug fixes
- #2446105 - Keep coded DNM source field mappings from overriding UI mappings.
- #2415977 - Use temporary:// instead of /tmp for drush logging.
- #2475473 - Fix handling of --idlist when map not joined.
- #2465387 - Fix handling of --stop option on migrate-import.