Filter icons look like proper buttons

The filter icons (Push events, Merge events, Comments, Team) now have a border
that signifies they can be pushed. This not only keeps it in line with the rest
of the application buttons, but it makes it more obvious, especially for the
Merge events button with a checkbox icon.

app/helpers/events_helper.rb

@@ -30,7 +30,7 @@ module EventsHelper
              end
 
     content_tag :li, class: "filter_icon #{active}" do
-      link_to request.path, class: 'has_tooltip event_filter_link', id: "#{key}_event_filter", 'data-original-title' => tooltip do
+      link_to request.path, class: 'btn has_tooltip event_filter_link', id: "#{key}_event_filter", 'data-original-title' => tooltip do
         icon(icon_for_event[key]) + content_tag(:span, ' ' + tooltip)
       end
     end