Beta welcome dialog

For configured wikis, show a dialog that welcomes the user to the
amazing and fantabulous world of VisualEditing, which is not only full of
wonderment and joy but also may lead to increased popularity and love.

The dialog only shows up once (uses a cookie).

Change-Id: I8e7c4dc2c63b36594378a543b9d66291395eebcf
Merge "Beta welcome dialog"