Add default focus outline to buttons

We were using a focus outline on links, but weren't doing the same for
buttons. Since sometimes browsers use a default outline which is barely
visible, this was very disorienting when browsing using the keyboard; we
were navigating through links that clearly indicated where the keyboard
focus was, and when reaching a button suddenly we had this almost
imperceptible feedback. Even if I'm used to it, my first reaction is
always "where did the focus go?" until I realize it's now on a button.

This is even more confusing because we've got buttons looking like links
and links looking like buttons.

Note that in the rules for the `:focus` styles we're including buttons
and the `[type="button"]` attribute. This seems redundant since those
styles are already covered by the `button` selector. However, Foundation
adds styles to buttons with the `[type]` attribute. Since the attribute
selector has precedence over the tag selector, we need to use the
attribute selector as well in order to override Foundation's styles.

app/assets/stylesheets/advanced_search.scss

@@ -40,10 +40,6 @@
       margin-bottom: 0;
       margin-top: $line-height / 4;
     }
-
-    &:focus {
-      outline: $outline-focus;
-    }
   }
 
   .general-search,