Split mixin/variable declarations from actual styling

[rmccue]

For better reuse, it'd be nice to have the mixins available without needing the default styles. For the Engineering site, I want to have control of the actual styles, but still want to use the common components (like buttons).

[rmccue]

This is specifically for typography.scss which a bunch of other stuff relies on, but buttons also relies on the a:link selector, which is not guaranteed to exist. Would also be nice to have that moved to a mixin :)