Some consistency tweaks in preparation for adding extension to translatewiki.net

* Fix type of extenson credits ('skin' is not defined)
* Remove translations which are identical to source
** The message keys will be set to 'optional'
* Kill trailing spaces

Change-Id: Ib425e34eab0e3aadd33645e66708b7c01864a520
Merge "Some consistency tweaks in preparation for adding extension to translatewiki.net"