Extract translations for log messages

Create for each log level its own translation file and update it
like the user visible translation file. Note that the debug log level
does not get translated and thus is excluded.

Take care that we handle empty pot files (transifex errors out on them)
and no initial existing pot files.

implements bp log-messages-translation-domain-rollout

Change-Id: Ib2f6f2450526655c41c27f411fb2868076049068
Merge "Extract translations for log messages"