Add tooltips for common Event feed entries

This adds tooltips to the event target for events like "opened merge
request !XYZ"

app/views/events/event/_common.html.haml

@@ -4,7 +4,7 @@
     = event_action_name(event)
 
   - if event.target
-    %strong= link_to event.target.reference_link_text, [event.project.namespace.becomes(Namespace), event.project, event.target], title: event.target_title
+    %strong= link_to event.target.reference_link_text, [event.project.namespace.becomes(Namespace), event.project, event.target], class: 'has-tooltip', title: event.target_title
 
   = event_preposition(event)