fixing header markup and some style tweaks

The two links are really buttons that perform actions. So don’t need
<nav>. Renamed relevant class names. Added clearer hover/focus styles
to buttons. Removed @aria-controls.
Can I please get you to change this to:

``` scss
.headerAction {
  background: none;
  border: none;
  color: $headerText;
  font-weight: bold;
  padding: 2px 1px 1px;
  text-decoration: underline;

  &:focus, &:hover {
    background: white;
    color: $primary;
    text-decoration: none;
  }
}

```

It'll compile to the same thing. Chur