Improve how slashes are stripped from filenames

Improve how slashes are stripped from filenames

* Add slash and backslash ('/' and '\') to $wgIllegalFileChars.
* Replace illegal chars before removing paths in wfStripIllegalFilenameChars().

This way users trying to upload a file with slashes in the name will
get a better filename suggestion (e.g. for 'Foo part 1/3.jpg', you
previously got '3.jpg', now you'll get 'Foo part 1-3.jpg'). Uploading
tools that don't special-case slashes will also behave better.

Change-Id: Ib78f48a5f8c92e8ab2dc773ea6789b96b3662177