Core should not use CSS classnames that contain an underscore

Currently the dashboard adds a class named "customize_mode"

Granted, the class is only used to check using JS:

if ($('#dashboard').hasClass('customize_mode')) {

but if some1 wanted to restyle things relative to that, this will fail in certain browsers unless escaped like

div.some\_thing { foo:bar; }

-> this example will look like this on IE6

attached patch changes customize_mode to customize-mode

note: this will effectively break #639172: Disable contextual links on the dashboard page while within customize mode and vice versa