Always render copyright notice in user language

All other footer content is rendered in the user language and the
copyright notice on the edit page is also in the user language.
See EditPage::getCopyrightWarning().

We already do this on WMF wikis through the WikimediaMessages
extension (which sets $forContent to false). The same behaviour has
been requested for 3rd party wikis as well.

Bug: 49116
Change-Id: I1dddfa7771c7063ec319fa441c2e400a374abc92