Add emphasis box component

[stephaniehobson]

This box with shadow and padding is showing up in designs enough that we should make it a
component:

[stephaniehobson]

See also mozilla/protocol-tokens#51

[johngruen]

is there a measurement spec for this or just a static image?

[amychurchwell]

@johngruen design linked in bedrock issue for page containing this component, also static image- mozilla/bedrock#7354 (comment) Should we track down sketch file? I personally feel that we have solid direction for the dev work here.