templates: Add "Download APK" button in apps.html.

This allows user to download the latest version of android apk from
the apps/android.

This will help the users who use Android without Google Play to
download the app and install it with ease.

To implement this I added a Download APK link on the apps.html page
which always points to the latest released version.

Fixes part of #11647.

static/js/portico/landing-page.js

@@ -110,6 +110,7 @@ var apps_events = function () {
     var update_page = function () {
         var $download_instructions = $(".download-instructions");
         var $third_party_apps = $("#third-party-apps");
+        var $download_android_apk = $("#download-android-apk");
         var version_info = info[version];
 
         $(".info .platform").text(version_info.alt);
@@ -130,6 +131,7 @@ var apps_events = function () {
             $third_party_apps.hide();
         }
 
+        $download_android_apk.toggle(version === "android");
     };
 
     $(window).on('popstate', function () {

static/styles/landing-page.scss

@@ -75,6 +75,7 @@ a:hover {
     text-decoration: none;
 }
 
+span a:hover,
 p a:hover,
 ul a:hover,
 ol a:hover {
@@ -2126,7 +2127,6 @@ nav ul li.active::after {
 
 .portico-landing.apps .hero .info button {
     padding: 10px 20px;
-    margin-top: 50px;
 
     font-size: 1em;
     font-weight: 600;
@@ -4251,3 +4251,10 @@ nav ul li.active::after {
         left: -7%;
     }
 }
+
+#download-android-apk a {
+    display: inline-block;
+    color: hsl(0, 0%, 100%);
+    font-size: 13px;
+    padding-left: 10px;
+}

templates/zerver/apps.html

@@ -26,6 +26,7 @@
                         <a class="link no-action" href="">
                             <button type="button" name="button" class="green">Download Zulip for <span class="platform"></span></button>
                         </a>
+                        <span id="download-android-apk"><a href="https://github.com/zulip/zulip-mobile/releases/latest">or manually download APK</a></span>
                     </div>
                 </div>
             </div>