[Frontend] CSS for better startup

Fixes #1966
- Transitions and opacity styles moved to new _app-start.scss,
which loads at the very end of all other CSS needed by the app;

platform/commonUI/general/res/sass/_app-start.scss

@@ -0,0 +1,11 @@
+// At the last, hide .l-splash-holder and show .holder-all
+.l-splash-holder.fadeout {
+    @include trans-prop-nice($props: opacity, $dur: 1000ms);
+    opacity: 0;
+    pointer-events: none;
+}
+
+.user-environ .holder-all {
+    opacity: 1;
+    pointer-events: inherit;
+}