Mention translatewiki.net on edits only, when edit a default message

When editing a page in the mediawiki namespace the hint to
translatewiki.net is always shown, but it is unneeded, when the message
is not a default message and maybe used for warnings in abusefilter or
is a gadget description.

Change-Id: I7dbf030311e30ce852459e363c91e988ca15d86d
Merge "Mention translatewiki.net on edits only, when edit a default message"