fix(filmstrip-only): first and last toolbar buttons get rounded edges

All toolbar buttons (which have the class .button) got wrapped
in additional elements due to atlaskit tooltips. This changed
parent/child relationships within the html hierarchy, and the
filmstrip only toolbar selectors needed updating.

css/_toolbars.scss

@@ -226,11 +226,11 @@
             width: 37px;
         }
 
-        .button:first-child {
+        .toolbar-button-wrapper:first-child .button {
             border-top-left-radius: 3px;
             border-top-right-radius: 3px;
         }
-        .button:last-child {
+        .toolbar-button-wrapper:last-child .button {
             border-bottom-right-radius: 3px;
             border-bottom-left-radius: 3px;
         }