Fix ScoreLog when a suggestion is accepted
Avoid of all submissions deletion cause of foreignkey

- set Submission.from_suggestion field to NULL value before deleting
pootle_app.suggestion objects.
- set Submission.suggestion field for those submissions that have
  type=SUGG_ACCEPT and don't have related suggestion (it needed to set
  Unit.reviewed_by data correctly)
Set Unit.reviewed_by and reviewed_on data based on Submissions
Use Suggestion.reviewer as submitter in accepting and rejecting submissions