Only transition border property

..since it's the only property which actually animates. (The background
image also changes on hover/focus, but that isn't visually effected by
transition, so we ignore it.)

This is for the next commit. Without this, the focus ring slowly
disappears, and the text slowly becomes the color it's meant to be;
neither of those are good.

src/components/forms/select.scss

@@ -7,7 +7,7 @@
     }
 
     select {
-        transition: all .5s ease;
+        transition: border .5s ease;
         margin-bottom: .75rem;
         border: 1px solid $active-gray;
         border-radius: 5px;
@@ -30,7 +30,6 @@
         }
 
         &:focus {
-            transition: all .5s ease;
             outline: none;
             border: 1px solid $ui-blue;
         }