Finished string extraction for i18n

Also a minor bug in preview for moderator has been fixed: The _moderator_userbar.html template contained the date in the "d M Y" format, which means that a string with the name of the month would be written. When the locale was 'el' this would contain a string with greek characters something that would leat to a 500-error when the wagtailuserbar would be called. This could be solved either by changign the date foramt to Y-m-d
(iso format, which is what i chose) or by adding a u before the '' in line 11 and 15 of wagtailuserbar.py.

Also all .po translation files till now have been pulled from Transifex and compiled.