⚠️ Rework completely how transitions works

- Use Svelte/Sapper native if and transitions to show either the page content or the loader, then load each page animationIn
- Code is safe on SSR side, using process.browser on this if
- The <main> element is on position absolute to fade nicely the different pages
- Code cleaning

src/style/layouts/_main.scss

@@ -1,5 +1,13 @@
 // Main page
 .housesof {
+    position: absolute;
+    top: 0;
+    left: 0;
+    right: 0;
     transition: opacity 0.4s $ease-quart;
     will-change: opacity;
+
+    &.is-transitioning {
+        opacity: 0;
+    }
 }