support "What's New" and "Video" fields from index-v1

fdroidserver currently only supports a single WhatsNew field that
comes from the CurrentVersionCode of the app.  Google Play and
fastlane supply support a WhatsNew field per-release, but we don't use
that data anywhere, and implementing that in the data structures would
add a lot of complexity since Apk would then need to have its own
"localized" section like App does.

The "Video" field is just a URL pointing to a video.

closes #910

app/src/test/java/org/fdroid/fdroid/updater/IndexV1UpdaterTest.java

@@ -251,6 +251,7 @@ public class IndexV1UpdaterTest extends FDroidProviderTest {
                 "upstreamVersionCode",
                 "upstreamVersionName",
                 "video",
+                "whatsNew",
                 "wearScreenshots",
                 "webSite",
         };