move rememberPad to cryptpad common and simplify it a bit

www/common/cryptpad-common.js

@@ -15,5 +15,35 @@ define([
         return secret;
     };
 
+    var rememberPad = common.rememberPad = window.rememberPad = function () {
+        // bail out early
+        if (!/#/.test(window.location.hash)) { return; }
+
+        var storageKey = 'CryptPad_RECENTPADS';
+
+        var recentPadsStr = localStorage[storageKey];
+
+        var recentPads = [];
+        if (recentPadsStr) {
+            try {
+                recentPads = JSON.parse(recentPadsStr);
+            } catch (err) {
+                // couldn't parse the localStorage?
+                // just overwrite it.
+            }
+        }
+
+        var now = new Date();
+        var timeframe = 1000 * 60 * 60 * 24 * 30;
+
+        var out = recentPads.filter(function (pad) {
+            return (pad && pad[0] !== window.location.href &&
+                (now.getTime() - new Date(pad[1]).getTime()) < timeframe);
+        });
+
+        out.push([window.location.href, now]);
+        localStorage[storageKey] = JSON.stringify(out);
+    };
+
     return common;
 });