Add mw-content-ltr|rtl to file redirect pages

Add mw-content-ltr|rtl to file redirect pages

28ea73bc3a07a4588f1b5fde4f5c41522eaa9518 added NS_FILE to the list,
because most of the content is user language. That does not
really apply for #REDIRECT, however.

As a side effect, this also makes the redirect arrows visible on File
redirect pages, because their CSS depends on these classes.

Another side effect is that now "additional" content will show for the
redirect as reported broken in T29857.

Bug: T60955
Change-Id: I6b14c17a4b74ff5964ea61554aa3afb197d206bf