Update Block component to be called Box again

[broccolini]

To do:

• rename Box to BorderBox
• rename Block to Box
• update all the places Box is currently used
• update all the places Block is currently used

[shawnbot]

Serious question: Are we planning on porting Box from Primer CSS? Because we use it a lot, both in the product and in our own docs. It seems kinda crazy to replace every instance of <Box> with:

<Block bg="white" border={1} borderColor="gray.2" borderRadius={1}>

[shawnbot]

FYI, I updated the todos in this after our discussion in Slack about Box being renamed BorderBox.