Update /apps screenshot for Windows and Mac.

(imported from commit 2886ffabb0fc7cea4ed30ffda4aba9ee8f5de03b)

templates/zephyr/apps.html

@@ -60,7 +60,7 @@
           <a href="https://humbughq.com/dist/apps/mac/Zulip-latest.dmg" class="btn btn-large btn-primary"><i class="icon-download icon-white"></i> Download Zulip for Mac</a>
         </p>
 
-        <img class="screenshot" src="/static/images/app-screenshots/humbug-desktop-mac.png">
+        <img class="full-width-screenshot" src="/static/images/app-screenshots/humbug-desktop-mac.png">
       </div>
 
       <div class="tab-pane" id="linux">
@@ -104,7 +104,7 @@ sudo apt-get install zulip-desktop
           <a href="https://humbughq.com/dist/apps/win/zulip-latest.exe" class="btn btn-large btn-primary"><i class="icon-download icon-white"></i> Download Zulip for Windows</a>
         </p>
 
-        <img class="screenshot" src="/static/images/app-screenshots/humbug-desktop-windows.png">
+        <img class="full-width-screenshot" src="/static/images/app-screenshots/humbug-desktop-windows.png">
       </div>
 
       <div class="tab-pane" id="plan9">

zephyr/static/styles/portico.css

@@ -125,6 +125,10 @@ img.screenshot{
     max-width: 150%;
 }
 
+.full-width-screenshot {
+    width: 100%;
+}
+
 .little-bullet {
     /* In our footer navbar, the · doesn't get appropriate padding without this */
     padding-left: 15px;