Move statistics updating outside the web browser code

At the moment, we have a lot of problems with translation statistics not being updated real-time even though we make every attempt to do so. I am suspecting this fails mostly due to timeouts in POSTs when we issue POFile.updateStatistics() call.

We have so far solved this with band-aids like cronscripts/rosetta-pofile-stats.py and cronscripts/rosetta-pofile-stats-daily.py, which have their own problems (bug 622668 and bug 781274). These scripts became even more important since message-sharing was introduced, because POFile.updateStatistics() updated the statistics only on a single POFile that was being touched, even though changes on its messages affect "sharing" POFiles as well. pofile-stats-daily script specifically has logic to find these "sharing" files, though it might be incomplete.

Instead of trying to fix our band-aids, we should fix the root cause instead. We know that sometimes it's going to be impossible to both save all the changes on a single +translate page *and* calculate the statistics (eg. ddtp-ubuntu templates are over 40k messages total) in a single POST, and we also know that statistics being very up-to-date real-time is not essential (we have gotten translators used to them being a week or so behind; having them eg. 5 minute behind at *most* would be a huge improvement).

So, we should instead decouple translation saving from statistics update. I propose we move any updateStatistics calls from the browser code (though, there should be only one) to a job which is scheduled to run whenever there are changes to a POFile. We should also extend updateStatistics call to support finding all the sharing POFiles as well (all the potentially touched POFiles).
The same should be done for POTemplate.newPOFile callsite which is called from within the browser code as well.

Other updateStatistics callsites (eg. in model/pofile.py and another one in model/potemplate.py importFromQueue) are related to imports which are already de-coupled from the browser code, so can (and should) stay "inline". Having them fix the statistics for all the sharing POFiles would be nothing but an improvement.

Once all of the above is done, we should be able to kill rosetta-pofile-stats-daily.py script, and leave rosetta-pofile-stats.py script around just in case we want to run it manually (we should remove it from the crontab or at least make is less common). Since it would go over all the POFiles, we should ensure that one doesn't try to find all the sharing POFiles when it's calling updateStatistics on one POFile.

Fixing this bug would solve bug 781274 (a critical regression) and bug 622668, thus marking this one as critical and adding notes there.