Delete data screen: Remove scrollbar when no convo selected

Turns out that the welcome screen had a min-height which caused weird
things when you take the window down to minimum height. It looked
particularly bad with the new full-screen delete data confirmation
screen.

background.html

@@ -85,10 +85,12 @@
     <div class='conversation-stack'>
       <div class='conversation placeholder'>
         <div class='conversation-header'></div>
-        <div class='content'>
-          <img src='images/icon_128.png' />
-          <h3>{{ welcomeToSignal }}</h3>
-          <p>{{ selectAContact }}</p>
+        <div class='container'>
+          <div class='content'>
+            <img src='images/icon_128.png' />
+            <h3>{{ welcomeToSignal }}</h3>
+            <p>{{ selectAContact }}</p>
+          </div>
         </div>
       </div>
     </div>

stylesheets/_index.scss

@@ -247,8 +247,16 @@ input.search {
 
 .conversation.placeholder {
   text-align: center;
+  .container {
+    position: absolute;
+    height: 100%;
+    width: 100%;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+  }
   .content {
-    padding: 100px 36px;
+    display: inline-block;
   }
 
   h3 {