Move to overlay scroll bars for the conversation list

Note that we're removing position: relative for the contact class, and
there might be some unexpected layout impacts. Thus far I haven't seen
any.

FREEBIE

stylesheets/_global.scss

@@ -91,7 +91,6 @@ button.hamburger {
 
 ::-webkit-scrollbar {
     width: 10px;
-    z-index: 1000;
 }
 
 ::-webkit-scrollbar-track {
@@ -317,7 +316,6 @@ $avatar-size: 44px;
   border-left: 4px solid $blue;
 }
 .contact {
-  position: relative;
   padding: 12px;
   white-space: nowrap;
   overflow: hidden;

stylesheets/_index.scss

@@ -24,7 +24,7 @@
   .content {
     background-color: $grey_l;
     flex-grow: 1;
-    overflow-y: auto;
+    overflow-y: overlay;
   }
 }
 .network-status-container {

stylesheets/manifest.css

@@ -105,8 +105,7 @@ button.hamburger {
     background-color: black; }
 
 ::-webkit-scrollbar {
-  width: 10px;
-  z-index: 1000; }
+  width: 10px; }
 
 ::-webkit-scrollbar-track {
   background: transparent; }
@@ -316,7 +315,6 @@ button.hamburger {
   border-left: 4px solid #2090ea; }
 
 .contact {
-  position: relative;
   padding: 12px;
   white-space: nowrap;
   overflow: hidden;
@@ -765,7 +763,7 @@ img.emoji.jumbo {
   .gutter .content {
     background-color: #f3f3f3;
     flex-grow: 1;
-    overflow-y: auto; }
+    overflow-y: overlay; }
 
 .network-status-container .network-status {
   background: url("/images/error_red.svg") no-repeat left 10px center;