Hygiene: Kill SkinMobile

Let's stop confusing people. One skin to rule them all.
Although the isMobileMode conditions might seem hacky
I'm confident that they will not be needed too frequently.

Kill addPageClass method - not used anywhere.
Remove .mobile class in favour of .skin-minerva
Add documentation to existing methods
Update css rules to hide .printfooter that now leaks from core

Change-Id: If323ee54e831c8e6dd0c2e8e668a30767d86b8f7