Better, retina-ready phone screenshots for /apps (though still not great).

(imported from commit 7a4a1fc0a203713eb83f75324f26abf8f890a37b)

static/styles/portico.css

@@ -126,6 +126,16 @@ img.screenshot{
     max-width: 100%;
 }
 
+.android-screenshot {
+    margin-top: -30px;
+    width: 221px;
+}
+
+.iphone-screenshot {
+    width: 184px;
+    margin: 0 32px;
+}
+
 .full-width-screenshot {
     width: 100%;
 }

templates/zerver/apps.html

@@ -19,10 +19,9 @@
     <div class="tab-content">
 
       <div class="tab-pane active" id="android">
-        <img class="screenshot pull-left" src="/static/images/app-screenshots/zulip-android.png">
+        <img class="screenshot android-screenshot pull-left" src="/static/images/app-screenshots/zulip-android.png">
 
-        <p><em>Note: The Android app is currently in alpha.</em></p>
-        <p>Zulip has a <strong>100% native app</strong> for Android,
+        <p>Zulip has a free, <strong>100% native app</strong> for Android,
           and you can easily grab it from
           the <a href="https://play.google.com/store/apps/details?id=com.humbughq.mobile">Google
           Play Store</a>.</p>
@@ -34,9 +33,7 @@
       </div>
 
       <div class="tab-pane" id="iphone">
-        <img class="screenshot pull-left" src="/static/images/app-screenshots/zulip-iphone.png" style="margin: 0px 32px;">
-
-        <p><em>Note: The iPhone app is currently in alpha.</em></p>
+        <img class="screenshot iphone-screenshot pull-left" src="/static/images/app-screenshots/zulip-iphone.png">
 
         <p>Zulip has a free, <strong>100% native app</strong> for iPhone and
         iPad. Please grab it from