Revert "An alternative (better) attempt to fix the mobile layout"

This reverts commit 846e84d.

.browserslistrc

@@ -1 +1 @@
-> 1%, last 2 versions, Firefox ESR
+> 1%

src/shared/components/challenge-detail/Header/style.scss

@@ -146,6 +146,10 @@
       flex-wrap: wrap;
       align-items: stretch;
 
+      @include xxs-to-xs {
+        flex-direction: column;
+      }
+
       .prizes-outer-container {
         flex: 4;
 

src/shared/components/challenge-detail/Specification/styles.scss

@@ -30,6 +30,10 @@ $tc-link-visited: #0c4e98;
   align-items: baseline;
   word-break: break-word;
 
+  @include xxs-to-xs {
+    flex-direction: column;
+  }
+
   .challenge-specifications {
     flex: 4;
     text-align: left;