Add refills hero, cards to welcome, purple colors

Specifically, I create a hero component and a cards component, borrowed
from refills. The hero component is pasted directly inside; cards
component is in the refills/ directory

Colors have been changed to a cool shade of purple, and variables in
_variables.scss have been updated accordingly.

Finally, the new hero image (to be replaced, perhaps) has been added to
assets, and the welcome controller has been updated accordingly