(bug 50007) Add 'Translation' namespace to English Wikisource

Adding this new namespace under no. 114 (115 for the corresponding
talk page namespace), enabling subpages for both and adding the
former to $wgContentNamespace. I'm also setting this new namespace
to be searched by default, all as requested in the bug.

Bug: 50007
Change-Id: Ib17903d6b85012f2297e117cdcfe5d82c61e4369