Enforce theme border radius

[fbarl]

We have different values appearing across our code, including 2px, 3px, 4px, 5px, 8px, 10px and 50%, but I think the only values we really need before enforcing the theme are 0, 4px (the current one) and 50%, e.g.:

borderRadius.none: 0
borderRadius.soft: 4px
borderRadius.round: 50%

The cases like border-radius: 0 4px 4px 0; could then be split into two lines:

border-top-right-radius: ${props => props.theme.borderRadius.soft};
border-bottom-right-radius: ${props => props.theme.borderRadius.soft};

cc @bia @jpellizzari