Added initial party-banner bar.

Mostly resolves ticket #882847 (https://bugzilla.mozilla.org/show_bug.cgi?id=882847), with the following remaining issues:
  * unseemly white gap under party-banner
  * map-canvas needs to take 100% of the remainder of the page (no scrollbars).

assets/css/events.less

@@ -1,6 +1,7 @@
 @import "mixins";
 @import "events/details";
 @import "events/map";
+@import "events/party";
 
 body#events {
     * { box-sizing: border-box; }

assets/css/events/party.less

@@ -0,0 +1,6 @@
+
+body#events {
+    #midbar {
+        background: #fff;
+    }
+}

views/events/map.html

@@ -2,8 +2,11 @@
 
 {% block title %}Event Map{% endblock %}
 
+{% block midbar %}
+<img src="/img/party-banner.png">
+{% endblock %}
+
 {% block body %}
-{{ super() }}
 <div class="map-overlay">
     <form id="find-event" action="/events" method="GET">
         <h2 class="header"><span class="icon-search"></span> Find an Event</h2>
@@ -102,3 +105,5 @@
 <div id="map-canvas"></div>
 
 {% endblock %}
+
+{% block footer %}{% endblock %}