images: Move operating system screenshots to app-screenshots directory.

Moves the three images of the web-app on different operating
systems (Mac, Ubuntu and Windows) to the same directory as
the two mobile-app screenshots, `/static/images/app-screenshots/`.
All five images are referenced in `static/js/landing-page.js`.

static/js/portico/landing-page.js

@@ -14,7 +14,7 @@ const hello_events = function () {
 const apps_events = function () {
     const info = {
         windows: {
-            image: "/static/images/landing-page/apps/microsoft.png",
+            image: "/static/images/app-screenshots/microsoft.png",
             alt: "Windows",
             description:
                 "Zulip for Windows is even better than Zulip on the web, with a cleaner look, tray integration, native notifications, and support for multiple Zulip accounts.",
@@ -24,7 +24,7 @@ const apps_events = function () {
             app_type: "desktop",
         },
         mac: {
-            image: "/static/images/landing-page/apps/macbook.png",
+            image: "/static/images/app-screenshots/macbook.png",
             alt: "macOS",
             description:
                 "Zulip on macOS is even better than Zulip on the web, with a cleaner look, tray integration, native notifications, and support for multiple Zulip accounts.",
@@ -52,7 +52,7 @@ const apps_events = function () {
             app_type: "mobile",
         },
         linux: {
-            image: "/static/images/landing-page/apps/ubuntu.png",
+            image: "/static/images/app-screenshots/ubuntu.png",
             alt: "Linux",
             description:
                 "Zulip on the Linux desktop is even better than Zulip on the web, with a cleaner look, tray integration, native notifications, and support for multiple Zulip accounts.",