stylesheets: Fix link styling

* Instead of changing the colour of links on hover to the highlight
  colour (which results in very poor contrast), use the box shadow
  effect that we already use in Framework and Lost-and-Found.

* For the login button on the header, change the cursor to a pointer
  so it appears as a link, instead of leaving it as an arrow since it
  actually is a button.  This helps POLA / UX.

Closes: ADA-668

Author: awilfox

app/assets/stylesheets/includes/main.scss

@@ -99,9 +99,10 @@ header {
     ul {
       a, form.calnet_auth > button {
         color: black;
+        cursor: pointer;
 
         &:hover {
-          color: $color-anchor-highlight;
+          box-shadow: inset 0 -4px 0 $color-anchor-highlight;
         }
       }
     }
@@ -119,7 +120,7 @@ footer {
     color: white;
 
     &:hover, &:focus {
-      color: $color-anchor-highlight;
+      box-shadow: inset 0 -4px 0 $color-anchor-highlight;
     }
   }