Ensure danger buttons have red hover state

074cb1a converted links that submit forms to buttons. To keep these
now-buttons looking like links it uses the `link` class. CSS looks for
buttons that are `not(.link)` to continue to style them appropriately.

However any `.button--danger` buttons did not get all the style
overrides. In particular, `:hover` and `:disabled:hover` states fell
back to the normal button background colors (blue).

This fixes those buttons to have the appropriate red hover states.

In addition, `.link--danger` elements also had a blue hover state. I'm
not sure when this was broken, or if it was always supposed to be like
that. Either way, I changed it so they now also appear slightly darker
when hovered to match the buttons.