Extend Servo's @page parsing to match Gecko's CSS 2.2 behavior, where only
margin properties are allowed in an @page rule. Other properties are ignored.
MozReview-Commit-ID: IPYUlnkLYSb
I've deliberately tried to keep things similarly structured as in the longhand files. I.e. if a given property is in e.g. longhand/box.mako.rs, the shorthand stuff that relates to the same property is in shorthand/box.mako.rs and so forth.
