Make .wb-snaktypeselector-menu visible on MonoBook

Menu for choosing "Unknown value" or "No value" was hidden because
MonoBook has div#content z-index: 2. Tested with Vector, MonoBook,
Modern & Cologne Blue.

Bug: 47942
Change-Id: I5fdcaaf78122dd8424dc96ea474deff4579c8632
Merge "Make .wb-snaktypeselector-menu visible on MonoBook"