Cache the interface language drop-down.
Sourround dynamic parts with nocache tags.

So that we can cache at action level too.
Fix the CurrentUser model.

So that we can use it from cached views too.
'null' is not a valid value for the $useTable attribute.
Removed caching of the statistics page.

There is no point into caching this page because:
• it would require a lot of versions, for instance 7 years of history
  means 7 years * 12 months * 19 interface languages = 1596 versions
• it’s disabled anyway