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

When creating submissions (and therefore ScoreLog entries), we need to
access the unit's state before it's saved, therefore we first create
the relevant submissions and then save the unit, adjusting the submitter
information when necessary.