accounts: fix infinite scroll over-loading, use scrollend+polyfill

calculation for number of rows to be drawn was wrong, fixed now. To
compensate for overshooting with fast scrolling, speed is calculated
using previous scrollY in rows/scroll, and used to render more rows.
Also, the "scrollend" event is used to load more at the end of a scroll
always. Since this isn't available on safari/webkit(2gtk), a polyfill
has been added.

ts/typings/d.ts

@@ -91,7 +91,7 @@ declare interface NotificationBox {
 
 declare interface Tabs {
     current: string;
-    addTab: (tabID: string, url: string, preFunc?: () => void, postFunc?: () => void) => void;
+    addTab: (tabID: string, url: string, preFunc?: () => void, postFunc?: () => void, unloadFunc?: () => void) => void;
     switch: (tabID: string, noRun?: boolean, keepURL?: boolean) => void;
 }