css: Start a separate stylesheet for "big screen" styles.

I'm about to add more and I *suspect* this will be easier to maintain
in the long term, rather than duplicating the conditional many times.

www/conservancy/static/css/conservancy-bigscreen.css

@@ -0,0 +1,12 @@
+/* "Big screen" means, approximately, bigger than a phone */
+
+.column:nth-child(odd) {
+    clear: both;
+    float: left;
+    width: 49%;
+}
+
+.column:nth-child(even) {
+    float: right;
+    width: 49%;
+}

www/conservancy/static/css/conservancy.css

@@ -262,18 +262,6 @@ h3 { margin-top: .6em; margin-bottom: .4em; }
 .column h2 a { text-decoration: none; color: #000000; }
 .column h2 a:hover { text-decoration: underline; }
 
-@media all and (min-width: 67em) {
-    .column:nth-child(odd) {
-        clear: both;
-        float: left;
-        width: 49%;
-    }
-    .column:nth-child(even) {
-        float: right;
-        width: 49%;
-    }
-}
-
 #conservancyfooter {
   margin-top: 1em;
   border-top: 1px solid #ccc;

www/conservancy/templates/base_conservancy.html

@@ -13,6 +13,7 @@
     <meta name="keywords" content="software, freedom, conservancy, open source, gnu, GNU, Open Source, Free and Open Source, Free and Open Source Software, FLOSS, FOSS, protect, protection, help, policy, linux, non-profit" />
     <link rel="shortcut icon" href="/favicon.ico" type="image/x-icon" />
     <link rel="stylesheet" type="text/css" media="screen" href="/css/conservancy.css" />
+    <link rel="stylesheet" type="text/css" media="(min-width: 67em)" href="/css/conservancy-bigscreen.css" />
     <link rel="stylesheet" href="/css/jquery-ui-1.8.22.custom.css" />
     <link rel="stylesheet" href="/css/jquery.ui.multiprogressbar.css" />
     <script type="text/javascript" src="/js/jquery-1.7.2.js"></script>