Use a list of links for admin stats events

Using <h3> headings for the links had two disadvantages.

First, it was the wrong heading level to use, since there was no <h2>
tag before it.

Second, headings are supposed to be followed by content associated to
that heading; here, we had no content following the headings.

So we're using a list of links and giving it a heading. We're adding
styles so the page still looks like it used to, although these styles
are certainly asking for improvements.

app/assets/stylesheets/admin/stats/event_links.scss

@@ -0,0 +1,14 @@
+.stats-event-links {
+  margin-top: $line-height;
+
+  ul {
+    @include header-font-size(h3);
+    font-weight: bold;
+    list-style-type: none;
+    margin-#{$global-left}: 0;
+
+    * + * {
+      margin-top: $line-height;
+    }
+  }
+}