D8 Port

Remaining tasks since Drupal Developer Days 2015 and all the work done after that (and continuing) :

• In the plugins configuration form, a validation error triggered with ajax is not removed when the form is re-submitted
• Check the log for probable undefined index issues
• We have too many createInstance plugin calls, we probably want to move those to a Utility Class (search in search_api code for ideas :))
• All Remaining non tested D7 features:
  • field collection attachment indexation: Needs test
  • entityreference attachment indexation: Needs test
  • ...
• Validate with http://pareview.sh/pareview/httpgitdrupalorgprojectsearchapiattachmentsg...