left sidebar: Fix scrollbar sticking up few pixels above container.

This is likely not the "right" fix in that it involved a negative
margin, but this does eliminate an annoying visual glitch where the
scrollbar overflows above its container in the left sidebar, without
creating other apparent problems.

Fixes #8731.

static/styles/left-sidebar.scss

@@ -46,7 +46,8 @@
 
 #stream_filters {
     overflow: visible;
-    margin: 2px 0px 22px 0px;
+    /* The -1px here prevents the scrollbar from going above the top of the container */
+    margin: -1px 0px 22px 0px;
     padding: 0;
     font-weight: normal;
 }