New features

* Upgrade core to 8.4.0
* Fix bug caused by upgrade

Thanks to all the community members pitching in to polish this up, especially @arosboro for porting redhen_dedupe in its entirety. This is an Alpha in name but for the features that are here it is certainly beta-quality or better. Beta is still waiting on some functionality, however.

Changes since 8.x-1.0-alpha2:

Changes since 7.x-1.0-beta4:

- Fixed and improved parts the language selection of all batch processes.
- Made the taxonomy manager work with localized and untranslated taxonomies as long as only a single language is selected + tried to keep LANGUAGE_NONE if required.
- Fixed a bug that potentially marks all term as updated during the first sync; Fixed a bug where the sync was marked as required even though the taxonomy was just exported into PP;
- Integrated the Background Process module for more usability for the batch processes.
- Fixed some minor bugs when background process is disabled.

- Added a PowerTagging mode selection that allows the user decide wether to use PowerTagging for annotation or for classification
- Improved the selection of the corpus in the backend.
- Added if statement for success in all the 'batch finished' process function to improve background process integration