Add reviewed_by and reviewed_on fields to Unit
Add ScoreLog

Every translation action (based on a Submission) has to be
saved as ScoreLog (part of #152)
Add score field to PootleProfile

part of #152
Log ScoreLog changes to pootle-activity.log

part of #152
Allow to use Unit.submitted_by as a previous translator

in Submission.save() method. Submissions are saved before Unit.save().
And current submitter is set to Unit.submitted_by field after Submission is
saved.