UX: Use Visual Viewport API for iOS composer height

This applies to iPhones running iOS 13+.
Previous technique remains in place for iOS 12 and below.

Note that this does not apply to iPads on iOS 13 due to Apple no longer
identifying iPads in the user agent string.

app/assets/stylesheets/mobile/compose.scss

@@ -21,6 +21,14 @@
     height: 250px;
     &.edit-title {
       height: 100%;
+      height: calc(var(--composer-vh, 1vh) * 100);
+    }
+  }
+
+  html.keyboard-visible &.open {
+    height: calc(var(--composer-vh, 1vh) * 100);
+    .reply-area {
+      padding-bottom: 0px;
     }
   }