Make remote content skin tabs non-File specific

Make remote content skin tabs non-File specific

This is one of the steps for implementing shadow namespaces -
generalizing foreign file specific code to be usable for any namespace.
This allows us to no-longer special case WikiFilePage in SkinTemplate
when building content_navigation, and will let GlobalUserPage to add
similar tabs just by overriding the same functions.

The new WikiPage::getWikiDisplayName() and WikiPage::getSourceURL()
functions will return expected values for both local and remote content.

Change-Id: Ib3d7dcbefe95da351872e63f306799eef83e00a7